NEW YORK — For decades, it was an article of medical faith: Get a mammogram; it could save your life.

Now, seemingly overnight, that faith has been shaken. The mammogram — that yearly ritual for millions of American women — has become the focus of a bitter and unusually public scientific dispute that is being fought in the pages of medical journals and the columns of daily newspapers.

The mammogram has always been a modest weapon, with benefits that women must weigh against possible risks. It is a screening tool that misses some tumors. At most, studies have found, it can cut the breast-cancer death rate by 30 percent.

Even when mammograms do "work," what they find does not always turn out to be cancer. The cancer they find may be growing so slowly that it would never threaten a woman's life. The result can be surgery, radiation and chemotherapy that are not medically necessary.

Over the years, scientists and statisticians have quietly debated the merits of mammography. Most of the public debate has focused on its effectiveness for women in their 40s.

That was already in doubt when the larger issue broke open last fall with the publication of a study by a pair of researchers based in Denmark. They argued that the clinical trials most often cited to support recommendations for mammograms were too flawed to be reliable.

Last month, an independent panel of experts at the National Cancer Institute agreed and said it could no longer make a recommendation about whether women should be screened.

The cancer institute said that it concluded that the new analysis did not refute evidence that mammography works, and that it is standing by its recommendation that women 40 and older be screened.

Many of those who did the original trials are defending their work. While there are flaws in the studies, they say, the Danish analysis exaggerated their significance and misinterpreted facts.

A number of experts agree.

"I think the trials have imperfections," said Dr. Steven Woolf, a member of the U.S. Preventive Services Task Force, a panel that reviews scientific evidence about disease prevention. "But the issue is whether they invalidate the studies. My own view is that they do not rise to that level."