“There are many cases of companies making unsubstantiated claims about the potential for stem-cell treatments to prevent, treat or cure serious diseases, and in those cases, we are committed to taking action and protecting patients,” an F.D.A. spokeswoman, Stephanie Caccomo, said in an email.

But in 2017 the agency gave stem-cell businesses a three-year grace period in which the F.D.A. has mostly looked the other way and waited for businesses to ask what rules applied to them and what they had to do to comply. The period ends in November 2020, but few businesses have responded.

Although F.D.A. officials have warned that once the deadline passes they will start cracking down, many past “enforcement actions” have consisted of warning letters without real teeth.

Leigh Turner, an associate professor at the University of Minnesota’s Center for Bioethics, called the court decision “meaningful.” But, he added, “It’s not going to automatically make hundreds and hundreds of businesses disappear. Some will decide they’re not up for a fight with the F.D.A.” But others may keep doing fat-based procedures, “and say, ‘The F.D.A. will have to reel us in one by one,’” he said.

In a recent study, Dr. Turner found 716 stem-cell clinics in the United States, a figure he calls a substantial undercount. The real total is probably more than 1,000, he estimated, and slightly more than half are selling fat-derived injections, like the ones given by U.S. Stem Cell.

The next most popular source of stem cells was bone marrow , which also lacks F.D.A. approval except for when it’s used to treat cancer or certain genetic diseases.

He suggested that some clinics might shift from using fat to bone marrow or other substances that they claim yield stem cells: umbilical-cord blood, the cord itself, or extracts from the amniotic membranes or placenta. Such treatments are also unproven and not F.D.A. approved (though cord blood is approved to treat leukemia).