The Web sites in the study generally portrayed their therapies as safe, effective and ready for routine use, but published research doesn't support that "overoptimistic" picture, the study authors said.
The analysis is presented in the December issue of the journal Cell Stem Cell by scientists at the University of Alberta in Canada. They cautioned that their overall findings can't be applied to the claims of any individual clinic.
The study is "a very important wake-up call," said Dr. George Daley, past president of the International Society for Stem Cell Research, who had no role in the new report.
"I think these Web sites are dangerous," said Daley, a Boston stem cell researcher. "They overpromise effectiveness and safety of the therapy and they completely underestimate and underinform about risks. ... (Such) overhyped marketing directly to the patient is putting patients at risk of financial exploitation at the very least, and physical danger at the worst."
In recent years, desperate patients with few options have traveled to China and other countries where doctors offer stem cell or other cell treatments for such things as spinal cord injuries, Parkinson's disease and blindness.
The new study did not assess the Web site claims directly by checking on how well patients actually fared at the clinics in the study.
Instead, researchers began with the 19 Web sites they found through Google in 2007. Treatments were promoted in several countries, including China, Mexico and Russia. None promoted treatments within the United States; one didn't give a location for treatments.
Last July, the researchers looked for published studies in human patients about using stem cells to treat the medical conditions mentioned most often by the Web sites: multiple sclerosis, Parkinson's and Alzheimer's diseases, spinal cord injury, stroke and heart attack.
They reported finding some encouraging hints but no clear evidence of benefit.
In the same issue of the journal, a report from the international stem cell society describes new research guidelines that condemn the marketing of unproven therapies. The society has posted a patient handbook on its Web site to help people who are considering stem cell therapy.
The guidelines say that in limited cases, doctors may be justified in trying an experimental treatment outside of a formal study for small numbers of seriously ill patients. The guidelines recommend standards for that situation, such as approval from a group of experts with no vested interest in the treatment and a commitment by those offering it to proceed to a formal study.