About 8:35 a.m. Wednesday, doctors in Pittsburgh and Philadelphia simultaneously began putting two completely healthy people under anesthesia.

Both patients had wanted to donate a kidney to a loved one, but instead, they ended up giving their gift of life by swapping places.

At Temple University Hospital in Philadelphia, 55-year-old Vyacheslav Komar of Greenfield was about to donate his left kidney to 46-year-old Mimi Weber of Lancaster, Pa., a paralegal and the mother of three boys.

At UPMC Montefiore Hospital, Mrs. Weber's sister, 51-year-old Wendy Weber of Wheeling, W.Va., was about to give up her left kidney for Mr. Komar's wife, 49-year-old Galina Komar, herself the mother of two sons.

By mid-afternoon, both recipients had their new kidneys in place, and on Friday, less than 48 hours later, all four of the patients met by videoconference, shedding tears, thanking each other, and talking about how remarkably good they all felt.

"We're truly blessed that it's gone so well -- we're two happy families," said Wendy Weber, who walked to her seat at the table and was preparing to go home that evening.

The so-called paired kidney exchange was the first done at either hospital and the first performed in Western Pennsylvania.

While these exchanges, which in rare cases involve three or four pairs of people, make up only a small fraction of kidney transplants, they are one of the fastest-growing procedures, and offer one more way to cut down on the huge transplant waiting list.

There are 78,000 people waiting for kidneys in the United States today, but only 16,628 transplants were carried out last year. Of those, a little more than a third came from living donors, almost all of whom were relatives or friends of the recipients.

But in 127 cases, the transplants were paired exchanges, in which one patient's relative or friend was a biological match for another patient, and that patient's relative or friend was a match for the first patient.

While that's less than 1 percent of the living donor kidney transplants, it's nearly four times the 34 paired procedures done four years ago.

For the Komars and the Webers, there were only four numbers that mattered last week, and all of them were looking good two days after surgery.

When Mimi Weber's kidneys failed three years ago from a rare autoimmune disorder known as IgA nephropathy, she went on overnight dialysis, and her sisters, Wendy and Heidi, immediately volunteered to donate kidneys to her.

But neither was a match. "We were just devastated," Wendy Weber said.

It was a similar story for Galina Komar, who works for UPMC Health Plan. The Belarus native has polycystic kidney disease, a progressive genetic disorder. When Vyacheslav, an auto mechanic, found out he wasn't a match for her, she said, he came home and said, "There should be some kind of program to exchange kidneys so we could exchange with another family."

Unbeknownst to him, there were four such exchange programs -- one in Boston, one at Johns Hopkins University in Baltimore, and two based in Ohio, including the one that UPMC and Temple have joined, the Paired Donation Network.

Dr. Ronald Shapiro, who transplanted Ms. Weber's kidney into Mrs. Komar, is president of the network, as well as director of kidney and pancreas transplants at UPMC's Thomas E. Starzl Transplantation Institute.

Eventually, many experts believe, the United Network for Organ Sharing, which oversees other organ transplants, should take over the kidney exchange networks as well, because the larger the pool of potential donors and recipients, the more matches that can be made.

One reason living donor kidney transplants have grown in number is because most of the donor surgeries are done using laparoscopy and minimal incisions.

Dr. Henkie Tan, who removed Wendy Weber's kidney and is UPMC's director of living donor transplantation, said he uses three small incisions to insert a fiber-optic camera and surgical instruments into the abdomen, and then performs the operation by looking at a plasma TV screen suspended above the bed.

After clipping off the ureter -- the tube that drains urine from the kidney -- he dissects it and the kidney artery and vein. He then inserts a plastic bag into the abdomen, maneuvers the kidney into the bag, and pulls it out through a 1 1/2-inch incision "below the bikini line."

The procedure is so minimal that Ms. Weber said Friday she felt almost no pain from the operation.

As doctors in the two cities were removing the left kidneys of Ms. Weber and Mr. Komar, other surgeons were creating an opening in the lower right abdomens of Mrs. Komar in Pittsburgh and Mrs. Weber in Philadelphia.

The timing is such, said Temple surgeon John Daller, that the donated kidneys "pretty much come out and are ready to fall back in" to the recipients' bodies.

Surgeons left the diseased kidneys in both patients' bodies and put the new kidneys in the front of their abdomens, attached to the iliac arteries and veins.

In both cases, the new kidneys immediately began cleansing the patients' blood of toxins. Mimi Weber's creatinine levels, a measure of those toxins, fell from more than 6 before the surgery to 2.2 on Friday, and Mrs. Komar's creatinine had gone from 5.8 to 1.9 in the same period. Normal creatinine levels are about 1.1

Dr. Steve Woodle, a founder of the Paired Donation Network and a University of Cincinnati transplant surgeon, said the network now has 200 pairs of recipients and potential donors listed in 12 states, and has done about two dozen paired exchanges.

"It's a very exciting time," he said. "We don't know for sure how this is going to shake out in the long run, but it's an exciting time."

For her part, Wendy Weber couldn't be more supportive. "I'm willing to tell anyone who will listen that the process is just not that bad," she said.

Next to her, Mrs. Komar said that Ms. Weber's equanimity should not discount what she had done.

"Not everyone can donate a kidney like you did," said Mrs. Komar, tearing up. "Not everyone has the personal power to do what you did for me."