Some clinics advertise treatments that seem to flout the regulations, offering, for example, stem cells from amniotic fluid, which do not qualify under the F.D.A.’s rules because they are not from the patient’s body. Others advertise stem cells collected from a person’s blood or fat to treat a neurological disease like Parkinson’s disease or multiple sclerosis. Those cells do not qualify because they do not normally function in the brain to control movement. There is no registry of patients going to these clinics, so it is unclear how many have been treated.

Outraged, Dr. Knoepfler has confronted some of the clinics.

“I have spoken to the clinics and they say, ‘O.K., you have lectured me about F.D.A. regulations, but I have been doing this for years and never heard a peep from the F.D.A.’ They see it as a tacit green light,” Dr. Knoepfler said.

On their websites, the clinics promote anecdotes and videos of happy patients. One clinic that uses stem cells to treat neurological diseases has a video of a patient, Barbara, and her husband on its website. She says she has multiple sclerosis and reports marked improvements after stem cell treatments, like driving again for the first time in years. “I feel so blessed that I have gotten better from the procedure,” she says.

Professional medical societies and state licensing boards should crack down, says Insoo Hyun, an ethicist at Case Western Reserve University and past ethics chairman for the International Society for Stem Cell Research (ISSCR). After all, he said, the stem cell clinics employ doctors, and doctors have a duty to offer patients evidence-based medicine.

But academic scientists feel stymied.

Dr. George Q. Daley, a stem cell researcher at Harvard Medical School and a past president of the International Society for Stem Cell Research, explained, “At ISSCR, we have spoken fairly aggressively about the false claims, but we have no enforcement power.” The group has its own website, A Closer Look at Stem Cells, that tries to caution patients, but Dr. Daley acknowledges it is no match for the clinic websites.

“The problem is a bit like the Whac-a-Mole game,” he said.

Some scientists want to counter the clinics by fighting anecdotes with anecdotes.

Anecdotes are undeniably powerful. We journalists use them constantly — nearly every story about a study of a new drug or treatment includes the obligatory patient, often with a photograph. Medical centers use them routinely in advertising — the grinning cancer patient praising a cutting-edge treatment, the person with a knee replacement crowing about how wonderful life is without constant pain. They are not the sort of evidence that would convince a scientist, but they are memorable in the way a study’s findings may not be.