BOSTON -- Each week, Leslie Gordon reviews the cases of every child in the world known to have progeria, a premature-aging disease.

These children almost always die of a heart attack, at an average age of 13. There are only 42 people in the world known to have the condition. One of them is Dr. Gordon's 10-year-old son, Sampson Berns.

"I now know more children who have died (of the disease) than are alive," says Dr. Gordon. "That is a turning point that I never wanted to reach."

Dr. Gordon, 42, has been helping to launch the first clinical trial of a potential treatment for the condition, one of the rarest diseases in the world. Doctors hope to begin enrolling children in the trial in March. But it is never far from her mind that time is short. The reality of the disease's progression means that even as she works to enable the children to join the trial, there is always the fear "we won't have enough kids at the end," she says.

Clinical trials for rare diseases are particularly difficult. Some companies don't want to test drugs for diseases where the market is small. It can be hard to find enough people to participate. Patient-advocacy groups often have to raise funds to help start trials and provide links between patients, doctors and companies.

The progeria trial, approved earlier this month by Children's Hospital Boston, faces a hurdle that plagues other rare diseases: With so few people who have the condition, there is no margin for error. In order to best determine if the drug makes a difference slowing down or reversing progeria, Dr. Gordon estimates the majority of children in the world with the disease -- about 25 of the 42 -- must join the trial. The trial will last at least two years. Many of the children are already between eight and 10 years old.

For the trial, children will fly to the Boston hospital from 15 countries, including Pakistan, Argentina and Venezuela. Many participants don't speak English and have never left their countries before. They will come to Boston every four months to see if an experimental drug may help them.

In getting families ready for the trial, Dr. Gordon, a clinical researcher, has spent hours answering medical questions, arranging for trial documents to be translated into Croatian, Korean and Urdu, and asking parents to send samples of their children's blood or baby teeth for study.

Her son Sam, born in 1996, appeared healthy at birth. She and her husband, Scott Berns, a pediatrician, bought a house in Foxborough, Mass., near her family. At first, they were so busy with their baby and jobs, they didn't even have time to unpack the boxes.

From the time Sam was 9 months old, there were subtle signs something was wrong. He wasn't eating well and his growth slowed. His teeth weren't coming in and his hair was thinning. His mother took him from doctor to doctor, but no one could figure out the problem.

One day, a friend from medical school came to visit. She had treated a child who had a progeria-like syndrome, and thought Sam looked similar. Increasingly worried about her son's condition, Dr. Gordon took leave from her job at a hospital. She and her husband took Sam to one of the few doctors in the country who specialize in progeria. In September 1998, Sam was diagnosed with the disease. He was 22 months old.

Progeria, which affects an estimated one in four million to one in eight million live births, is the result of a mutation in a gene that causes the children to make an abnormal form of protein. Buildup of that protein leads to many of the problems associated with the disease -- including heart attacks and strokes.

At first, Dr. Gordon took long car rides where she could scream or cry in private. "I was in mourning," she says.

Within weeks, she had taken all the boxes out of the study, and turned it into an office. In 1999, Dr. Gordon, her husband and her sister, Audrey Gordon, started the Progeria Research Foundation.

The foundation worked to find ways for the few families in the world with progeria to connect. The doctor who diagnosed Sam knew many of the children in the U.S. who have the disease. There was a group of patients in Europe. Most found the foundation over the Internet. The foundation also set up a private online message board for the families to use and links from its Web site to the personal pages that other families created about their own children.

Dr. Gordon and Dr. Berns met with Harold and Suzette Kushner, whose son, Aaron, died of progeria in 1977 at the age of 14. Rabbi Kushner's book, "When Bad Things Happen to Good People," was inspired by his family's experiences. Back then, it was almost impossible to find other families who had children with progeria. Before Aaron died, Rabbi Kushner recalls, he told his parents he wanted to do three things: visit the National Baseball Hall of Fame and Museum, see an R-rated movie, and meet another child who had progeria. "We did the first two," he says. "We couldn't do the third."

In June 2000, the foundation convinced 15 families of children with progeria to join them on Capitol Hill at an ice-cream social with senators and members of Congress. A few months later, when the Children's Health Act was passed, language in the bill required the National Institutes of Health to do something to support children with the disease.

One reason researchers are intrigued by such an extremely rare disease is that they believe it could hold clues to the aging process. Insights into heart attacks -- which kill virtually all children with progeria -- are of broad interest because they affect so many adults.

But the notion that progeria might offer a potential model for studying aging wasn't firmly established until after some of the initial studies financed by the Progeria Research Foundation yielded tantalizing clues. So often in rare diseases, it is up to the patients and their families to raise the money and find the researchers who can make important discoveries that then start attracting additional people into the field.

Francis Collins, director of the NIH's National Human Genome Research Institute, met Sam's parents and eventually agreed to help look for the gene that causes progeria. "I put a picture of Sam next to my computer," he says. "Every evening I came home from work and I looked at that picture of Sam and I said to myself, 'Another day has gone by and we have not yet come up with a solution. We have to hurry up.'"

So researchers would have enough to study, the foundation set up a cell and tissue bank. Working late into the night in her wood-paneled home study, Dr. Gordon sent emails, wrote letters, and made calls to parents all over the world, asking them to send their children's blood samples, skin biopsies and even baby teeth.

In 2003, researchers discovered the progeria gene. Until researchers knew what gene was involved, they didn't know what kind of drugs, if any, might reverse or stop the disease. A drug called a farnesyltransferase inhibitor, or FTI, in experiments stopped further production of the abnormal protein that causes the problems faced by children with progeria.

Several companies make FTIs, which are being tested to fight cancer. Researchers used the drug to reverse abnormalities in progeria cells in the lab, and in mice they were able to prevent some of the disease's symptoms by treatment with FTIs. That set the stage for the progeria trial, which will use an FTI developed by Schering-Plough Corp., of Kenilworth, N.J.

The foundation began to raise the $2 million for a trial, including the cost of flying in the families. About $1 million has been raised so far.

In September, at the Progeria Research Foundation's annual 5K race in Peabody, Mass., Dr. Gordon sat on the steps of City Hall, chatting with Thomas and Kathleen McGuinness. The couple were going to hold their own fund-raiser, in memory of their son, Kristian, who died of a progeria syndrome last year, at age 3. They wanted to know if Dr. Gordon's son Sam would be coming.

Dr. Gordon thought it would be too painful. Sam had known other children who died of progeria, and every time, Dr. Gordon saw the pain and the questions about his own fate. "It's hard on him," she says. "He feels all of it."

Each death was a reminder of the time pressure Dr. Gordon faced. Kristian's father says they had made arrangements to fly with him for tests at NIH to better understand progeria. "He was supposed to fly there three days after he passed away," Mr. McGuinness says.

There wasn't always advance warning. A very young child could suddenly have a stroke. In August, an 11-year-old with progeria, who had no apparent evidence of cardiovascular problems, suffered a massive heart attack and died after jumping on a trampoline. "Every child is on his or her own trajectory to a stroke or a heart attack," Dr. Gordon says.

In October, Dr. Gordon told the foundation's board about her progress. She had ordered 30 scales, and families interested in the trial were recording their children's weight changes. Once the trial started, one measure of whether the drug was effective would be if the children gained weight. Every week, parents called or emailed the data to the foundation.

The recent deaths hung over her. "They stay on my mind everyday," Dr. Gordon told the board. When her husband presented a new video for fund raising -- featuring pictures of children, including Sam -- Dr. Gordon couldn't watch. She quickly left the room.

At the foundation's small office, a map of the world hung on the wall. There were blue pins marking each country where a child had died, and red pins for children who were alive. The map was out of date; they needed to put in 10 more blue pins, Audrey Gordon said.

Even with diseases that affect many people, it can take a long time to fill slots in a trial. In a trial for early-stage breast cancer, open at hundreds of sites around the country, doctors need 4,660 women, but only 40 to 50 are signing up a month. They say it will take three times longer than expected to complete the trial. A trial trying to test a way to prevent lung-cancer recurrence started almost a decade ago. It needs 2,000 participants; so far, 1,250 have signed up. "At this rate the study will not be completed until the year 2010," says David H. Johnson, deputy director of the Vanderbilt-Ingram Cancer Center in Nashville, Tenn.

Mark Kieran, a pediatric oncologist at the Dana-Farber Cancer Institute in Boston who will be principal investigator on the progeria trial, understands Dr. Gordon's concerns about the pace. "We are moving at lightning speed," he says. "But Leslie thinks we are tortoises."

As much as Dr. Gordon checked her list of children, she couldn't be absolutely certain how many would enroll. A health crisis, a death, an unexpected change of heart, could shift the balance of the trial. "I just never know," she says.

The parents of Seth Cook, 13, were excited about the trial. But last summer, Seth had a stroke. The family lives near Seattle, and the idea of having to travel to Boston so frequently when Seth is weak seemed overwhelming. "He's still not up to par to subject him to a test," says Kyle Cook, Seth's father.

Jamie Scullion, whose 10-year-old son Devin has progeria, was also following the growing possibility of a trial. "Before I met Leslie, I knew there was nothing. I accepted the fate of my child," she says.

Devin's health is fragile. He weighs less than 24 pounds and has suffered two strokes. He uses a walker while at school, and takes blood thinners, Lipitor and baby aspirin in the hopes of preventing a heart attack and more strokes. "He hates taking those pills," his mother says.

When she first broached the idea of the trial with him, he asked, "Does this mean I have to take another pill?" It does: Devin will take the drug in pill form twice a day.

She now says she plans to enroll Devin, even though it means flying from Ontario. "I hope it gives him more time, and more good days," she says.

At Dr. Berns and Dr. Gordon's home, Sam has built an elaborate city out of Legos that he calls Royal Base. On a recent day, when he dropped a Lego piece on the floor, he picked it up with his toes. At the age of 10, it was painful for him to bend over because of joint contractures and other bone problems associated with progeria.

He scooted back and forth to the basketball net hanging on his bedroom door. He spoke about wanting to be an inventor and has an idea for building a "hovercar" that could fly over traffic. He said he wants to go to MIT for college.

The focus on the future comes from Sam, his parents say. "I never would have said to him, 'Where do you want to go to college?'" Dr. Gordon says. "He makes an assumption that I left alone personally." Her husband added: "Birthdays are bittersweet."

In thinking about whether the trial could result in a treatment to help Sam, Dr. Gordon says, "It is so damaging to over-hope. You have to stick to reality." She thinks the drug could help preserve his quality of life and lead to him living longer. "But it doesn't translate into erasing the disease," she said.

In 2006, Dr. Gordon won an award from Working Mother magazine and the family went for a breakfast in New York. Sam sat at a table with actress Susan Sarandon and chatted with her.

"I was giving this very dramatic speech trying to explain progeria to the world," recalls Dr. Gordon.

Sam asked to go up to the podium when his mother was done, because he had something to say. One day, he told the audience, he was sledding. He kept jumping off midway. The hill got steep and the sled went too fast and he was scared he would fall. His mother had an idea. On his next run, she grabbed the sled, hoping to slow him down so he could continue to the bottom of the hill. Instead, Sam went flying and landed face down in the snow. He ended up with a scar on his nose, which he pointed out.

After Sam finished, the audience burst into applause. Dr. Gordon's speech had been about the science driving the trial; Sam's got them to identify with its precariousness. The effort had always been about trying to ease the children's path down the hill. Sam's story was a reminder that it was never certain if the child was going to make it safely to the bottom, or was about to go flying through the air, out of reach.