It could have been a sign that the man was infected with H.I.V.; the only way to tell was further testing. But Dr. Chinnaiyan, who leads the Center for Translational Pathology at the University of Michigan, was not able to suggest that to the patient, who had donated his cells on the condition that he remain anonymous.

In laboratories around the world, genetic researchers using tools that are ever more sophisticated to peer into the DNA of cells are increasingly finding things they were not looking for, including information that could make a big difference to an anonymous donor.

The question of how, when and whether to return genetic results to study subjects or their families “is one of the thorniest current challenges in clinical research,” said Dr. Francis Collins, the director of the National Institutes of Health. “We are living in an awkward interval where our ability to capture the information often exceeds our ability to know what to do with it.”

The federal government is hurrying to develop policy options. It has made the issue a priority, holding meetings and workshops and spending millions of dollars on research on how to deal with questions unique to this new genomics era.

The quandaries arise from the conditions that medical research studies typically set out. Volunteers usually sign forms saying that they agree only to provide tissue samples, and that they will not be contacted. Only now have some studies started asking the participants whether they want to be contacted, but that leads to more questions: What sort of information should they get? What if the person dies before the study is completed?

The complications are procedural as well as ethical. Often, the research labs that make the surprise discoveries are not certified to provide clinical information to patients. The consent forms the patients signed were approved by ethics boards, which would have to approve any changes to the agreements — if the patients could even be found.

Sometimes the findings indicate that unexpected treatments might help. In a newly published federal study of 224 gene sequences of colon cancers, for example, researchers found genetic changes in 5 percent that were the same as changes in breast cancer patients whose prognosis is drastically improved with a drug, Herceptin. About 15 percent had a particular gene mutation that is common in melanoma. Once again, there is a drug, approved for melanoma, that might help. But under the rules of the study, none of the research subjects could ever know.