Microscopic embryonic stem cells might hold the answer to afflictions from baldness, wrinkles, and age spots to the most intractable diseases.

Or they might be a medical mirage, a tempting oasis of healing on a horizon that never grows closer.

This fall, as entities ranging from California to the United Nations prepare to make major decisions about the future of stem-cell research, American public opinion has swung strongly in favor of the technology. But just as medical research has created breakthroughs in physical treatments, it has also led doctors - and the public - down blind alleys.

It's not yet clear which path stem-cell research will follow. It is, however, certain that it will spark more political fireworks than any medical technology in at least a quarter century.

"Americans love medical technology," says Robert Blendon, a professor of health policy and management at Harvard University who tracks public attitudes toward medicine. But embryonic stem cells have become linked to the abortion debate - "the single biggest, most contentious issue in American society," he adds. That alone makes the issue unique.

To be sure, those in the field are pretty sure they're onto something exciting. "There's absolutely nothing we've seen so far to suggest that this cannot be done," Dr. Douglas Melton, a leading stem-cell researcher at Harvard University, said at an international conference on stem-cell research in Boston in June.

But others remember a cautionary tale from the mid-1990s: "gene therapy" aimed at replacing, manipulating, or supplementing human genes that were not working with healthy genes.

"When it [gene therapy] was first proposed, it was going to be the way to fix everything," says Josephine Johnston, a bioethicist at the Hastings Center, a nonprofit bioethics research institute in Garrison, N.Y. Advocates announced that treatments based on genetic manipulation would be available no later than the turn of the millennium. But by 2004, not one had reached the market, while many human test subjects had suffered adverse effects. Several died.

For the moment, US public opinion is moving in favor of stem-cell research, especially after pleas from the wife and son of former President Reagan, whose battle with Alzheimer's disease raised the question whether the nascent technology might have provided a cure.

The swing in the polls is a boon for proponents, because major decisions about the technology's future loom. In October, the United Nations General Assembly is expected to vote whether to adopt a ban on all human cloning, backed by the Bush administration. Or it might ban only reproductive cloning and allow so-called therapeutic cloning, which creates stem cells.

On Nov. 2, Californians will vote on a ballot initiative to provide $3 billion in state funds for stem-cell research. Proponents, including billionaire Bill Gates, have contributed more than $10 million to push its passage, suggesting that it will be a boon to the state and lead to breakthroughs that the federal government has refused to underwrite.

Supported by antiabortion advocates, the Bush administration has restricted federal funds for embryonic stem-cell research to a series of stem-cell lines created before Aug. 9, 2001. (Scientists are also studying stem cells from adults and the blood from the umbilical cords of newborns. While showing promise, these may have limited therapeutic abilities.) Fewer than 20 of the original 78 embryonic stem-cell lines have proven useful to scientists, who argue they need access to many, many more to conduct meaningful research.

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