On the other side are those, especially 23andMe executives, who argue that the services merely provide information, not medical diagnosis, and that consumers have a right to the information contained in their genes. At a time when consumers are taking more control of their health care, denying them such information would be, as one director of 23andMe recently put it, “appallingly paternalistic.”

The companies have also said that their tests do not require F.D.A. approval because they have been developed and are offered by a single laboratory. The F.D.A. has typically refrained from regulating such tests, as opposed to test kits that are widely sold to laboratories, hospitals and doctor’s offices.

The F.D.A. is now clearly deciding in favor of regulation, saying the tests of disease risk can have medical consequences.

“It is not unknown for women to take out their ovaries if they are at high risk of ovarian cancer,” Dr. Gutierrez said. Some of the services are also offering consumers information that could be used to determine the doses they should get of particular drugs, like the blood thinners, warfarin and Plavix.

But Dr. Gutierrez denied that the agency was being paternalistic. “We really don’t have any issues with denying people information,” he said. “We just want to make sure the information they are given is correct.”

In a statement Friday, 23andMe said it disagreed with the F.D.A.’s conclusion but was open to discussion on ways to regulate the personal genetics industry. “We are sensitive to the F.D.A.’s concerns, but we believe that people have the right to know as much about their genes and their bodies as they choose.”

By contrast, Kari Stefansson, head of research at DeCode, said the interest by the F.D.A. was “timely, appropriate and welcome” because the company wanted its test to become part of standard medical care. A spokesman for Knome said the company would cooperate with the F.D.A.