A decade ago, Denise Shields of Laguna Niguel, Calif., and Nancy Hinkeldey of Orlando, Fla., went to China with a group of other Americans intent on adopting.

They were among 10 families, from the East Coast, Texas, Ohio and California, who had all received "referrals" from Chinese authorities to adopt children in Nanning, in the southern province of Guangxi.

Shields, 49, had lost her husband in an accident in 1987 and was aching to raise a child. Hinkeldey, 48, a single mother with one adopted child, wanted another. Their mutual yearning was satisfied Nov. 7, 1994, at an orphanage in the southern city of Nanning. Each flew home with a baby girl.

Over the years, Shields sent holiday photos of her daughter to the other families who had made the trip. As the photos piled up, Hinkeldey began seeing subtle similarities between Emily Shields and her own daughter Anna. She saw it in their jaws and cheek lines, in their eyebrows and foreheads, in their hairlines.

Last July, as the girls romped on the beach at their travel group's 10-year reunion on Amelia Island, Fla., the mothers compared notes.

Both girls had been late losing their baby teeth. Both were academically gifted, musical and artistic. Then Denise Shields noticed a peculiar indentation on the back of Anna's right thigh. She grabbed Hinkeldey's arm and blurted out: "Anna has the same mark Emily does!"

The two women discussed with their daughters the idea of DNA testing. Anna and Emily agreed. The news came in August: The girls shared at least one birth parent, making them biological half-sisters. The lab could not rule out the possibility that they had both parents in common.

DNA testing has been used for years to convict felons and clear those who've been wrongly accused. Now, families are turning to it to establish biological links between adopted children living in different households, often hundreds or thousands of miles apart.

In August, a company in Princeton, N.J., announced that it had agreed to conduct genetic testing for a nonprofit registry that is believed to be the first to focus on using genetic profiles to reunite siblings after adoption. Other such registries are scrambling to enter the market.

The movement is stirring up a commotion in adoption circles. Some adoption specialists and bioethicists say they fear that unscrupulous labs will exploit parents eager to fill in the missing pieces of their children's lives.

Another worry is that adopted children could find the sudden discovery of a sibling profoundly unsettling.

Bioethicists and educators also caution that DNA tests conducted without genetic material from the parents can often prove inconclusive.

"Before we all go flying off getting our DNA tested, we need to have some thought given to what is going to happen when we get these results and how we'll interpret them," said Jane Gitschier, a professor of medicine and pediatrics at the University of California, San Francisco.

But Hinkeldey and Shields believed they owed it to their daughters to find out whether the similarities between them were accidental or hereditary.

"We felt it would be doing them a disservice not to," Hinkeldey said.

In the decade since their adoptions, the 10 girls taken in by members of the Nanning travel group had come to calling themselves "the China Sisters," connected by a bond their families believed to be as deep as any biological tie.

As Emily grew, she pined for a sister. In Florida, Anna sometimes felt lonely, despite having an older and, later, a younger sister, both also adopted from China. "I felt like a little part was missing," Anna would say later.

Hinkeldey had heard scattered reports of adoptees' discovering blood connections through DNA testing. Soon after last summer's beach reunion, she began searching the Internet for information.

She found an e-mail group called Sister Far, started by Susan and Jim Rittenhouse of Lisle, Ill. Participants include the families of more than 60 suspected or confirmed sets of twins and other siblings, most from China but also from Cambodia, Russia and Nepal.

The families, who live in the United States, Europe and Australia, share news and tips and discuss how to deal with issues that can arise when families are suddenly thrown together by the possibility -- or the certainty -- that their children share bloodlines.

Jim Rittenhouse said the couple started Sister Far earlier this year after tests indicated "a strong certainty" that their daughter Meredith Grace was the biological sibling of a girl in Alabama whose spitting-image photo they had seen on an e-mail list for families with children from Jiangmen. That child, coincidentally, was named Meredith Ellen.

Through Sister Far, Shields and Hinkeldey encountered stories that were positive, even joyous. The reports encouraged the two mothers to proceed with DNA testing.

Like other parents in their situation, Hinkeldey and Shields set out to become experts in a field in which authorities of long standing disagree on important points, such as how many genetic markers must be tested to achieve reliable results.

Shields, a business consultant, heard from a client about Genex, a lab with the Web site www.swabtest.com. Shields said the company cautioned that, without genetic material from the biological parents, test results could prove ambiguous. Some full siblings, for example, might not share any of the tested markers.

To complicate matters, even biologically unrelated individuals might by chance share a number of markers.

The two women paid $145 each for a "siblingship" test. They rubbed the insides of their daughters' cheeks with cotton swabs and sent the packets to the lab.

Two weeks later, Hinkeldey and her daughters flew to New Hampshire for a family reunion. While the Hinkeldeys were in the air, Shields called Genex to ask whether the test results were available.

A lab employee told Shields that the girls were "at least half-siblings."

Shields asked: "Are you saying they are sisters?"

The answer: "Yes."

Shields brought her daughter into her home office and shared the news. Next, she called her parents. Then, she tried Hinkeldey's cell phone dozens of times before finally reaching her just as she got off the plane. On hearing the news, Hinkeldey screamed. The whole family celebrated.