On the surface, the claim is plausible. Scientists have discovered that fat, bone marrow and other parts of the body contain stem cells, immature cells that can rejuvenate themselves, at least in the tissue they are naturally found.

But it has yet to be proved that these cells can regenerate no matter where they are placed, or under what conditions this might occur. Moreover, questions about safety remain unanswered.

These sober realities do not appear to have slowed the rise of an international industry catering to customers who may pay tens of thousands of dollars in cash for their shot at a personal miracle. (Some foreign operators offer creative variations on the theme, like cells from sharks and sheep.)

Domestic providers, too, can push the limits. In July, for example, a former pathologist at the Medical University of South Carolina pleaded guilty to illegally processing and shipping stem cells for treatment without approval from the university or the Food and Drug Administration.

The number of clinics and products has reached the point that scientists fear repercussions for their own work. Dr. Hesham Sadek of the University of Texas Southwestern Medical Center in Dallas, who is studying heart muscle regeneration, worries that the marketing deluge now makes it hard for patients to tell science from swindle, and all that lies on the spectrum in between.