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The genetic data of more than 1,000 people from around the world seemed stripped of anything that might identify them individually. All that was posted online were those data, the ages of the individuals, and the region where each of them lived. But when a researcher randomly selected the DNA sequences of five people in the database, he not only figured out who they were, but he also identified their entire families, though the relatives had no part in the study. His foray into genomic sleuthing ended up breaching the privacy of nearly 50 people.

And all it took was triangulation, using the genetic data, a genealogy Web site and Google searches. While the methods for extracting relevant genetic data from the raw genetic sequence files were specialized enough to be beyond the scope of most laypeople, no one expected it would be so easy to zoom in on individuals.

"We are in what I call an awareness moment," said Eric D. Green, director of the National Human Genome Research Institute at the National Institutes of Health.

The researcher did not publish the names he found. But the exercise revealed a growing tension between the advancement of medical research, which often requires making genetic information public so scientists can use it, and protecting the privacy of study subjects.

The paper, published Thursday in the journal Science, follows other reports that identified people whose genetic data were online. But none had started with such limited information: just the long string of DNA letters, an age and, because the study focused on only American subjects, a state.

"I've been worried about this for a long time," said Barbara Koenig, a researcher at the University of California in San Francisco who studies issues involving genetic data. The new paper is "amazing," she added, but "we always should be operating on the assumption that this is possible."

There is no easy answer about what to do. Make study subjects more aware that they could be identified by their DNA sequences? Keep more data locked behind security walls? Institute severe penalties for those who invade the privacy of study subjects? None of the above?

"We don't have any claim to have the answer," Dr. Green said. And opinions about just what should be done vary greatly among experts.

But after seeing how easy it was to find the individuals and their extended families, the N.I.H. removed people's ages from the public database, making it more difficult to identify them.

But Dr. Jeffrey R. Botkin, associate vice president for research integrity at the University of Utah, which collected the genetic information of some research participants whose identity was breached, cautioned about overreacting. Genetic data from hundreds of thousands of people has been freely available online, he said, yet there has not been a single report of someone being illicitly identified. He added that "it is hard to imagine what would motivate anyone to undertake this sort of privacy attack in the real world." But he said he had serious concerns about publishing a formula to breach subjects' privacy. By publishing, he said, the investigators "exacerbate the very risks they are concerned about."

The project was the inspiration of Yaniv Erlich, a human genetics researcher at the Whitehead Institute, which is affiliated with M.I.T. He stresses that he is a strong advocate of data sharing and would hate to see genomic data locked up. But when his lab developed a new technique, he realized he had the tools to probe a DNA database. And he could not resist trying.

The tool allowed him to quickly find a type of DNA pattern that looks like stutters among billions of chemical letters in human DNA. Those little stutters — short tandem repeats — are inherited. Genealogy Web sites use repeats on the Y chromosome, the one unique to men, to identify men by their surnames, an indicator of ancestry. Any man can submit the short tandem repeats on his Y chromosome and find the surname of men with the same DNA pattern. The sites enable men to find their ancestors and relatives.