ASHINGTON, March 14 — A new analysis of mammography, the latest to address the question of whether screening for breast cancer saves lives, has found that the tests reduce the risk of dying from the disease by one-fifth.

The study, made public today by a Swedish team, said the benefits of screening were greatest for women older than 55. Among younger women, the benefits were not statistically significant, the team wrote.

The research is unlikely to settle the debate among scientists and statisticians over the value of mammograms. But proponents of mammography said the study bolstered their case, adding that the overall findings lent credence to the government's recent recommendation that women older than 40 have the tests every one to two years.

"It adds yet more evidence to suggest that mammography screening reduces mortality," said Dr. Steven Woolf, a professor of family medicine at Virginia Commonwealth University who helped draft the government recommendations. "It goes onto a pile of existing studies showing that mammography is effective."

Routine mammograms have been a part of the lives of American women for decades, but the debate on their usefulness has gone on for years. At issue is whether the benefits of detecting tumors early, when they are small and can be easily removed, outweigh the risks, which include false positive test results and unnecessary surgery to remove tumors that might not be dangerous.

The Swedish study is an updated analysis of four large Swedish clinical trials that have been at the center of that debate. In October, scientists in Denmark announced that they had found serious flaws in studies, including the Swedish trials, that have been widely cited as proof of the benefits of mammography.

The new review is, in effect, the Swedish rebuttal. Led by Dr. Lennarth Nystrom of Umea University, the scientists extended their follow- up of 247,000 women in the four Swedish studies and re-examined an earlier analysis in light of the Danish critique.

"The advantageous effect of breast screening on breast cancer mortality persists after long-term follow-up," Dr. Nystrom and his colleagues wrote. They called the Danish critique "misleading and scientifically unfounded."

The study, which appears in the current issue of the journal Lancet, found that regular mammograms reduced breast cancer deaths among women of all ages by 21 percent. When the authors adjusted their data to account for criticism by the Danish group, that figure changed to 15 percent.

The study found that the benefits of screening became statistically significant beginning at 55. Among 55- to-64-year-olds, for instance, deaths among women who were offered screening were reduced 27 percent. By comparison, the deaths were reduced 14 percent among 45-to-54- year-olds who were offered screening.

Because the original studies intended to look at the benefits of mammography overall, and not among particular age groups, a number of experts today said that it was wrong to conclude that screening did not benefit younger women. The correct interpretation of the data, they said, is that the benefits of mammography increase with age.

"There's a continuum," Dr. Woolf said. "As women get older, the tradeoff of benefit to harm becomes more favorable. There's nothing magical that happens" at 55.

In an editorial accompanying the research, Dr. Karen Gelmon of the British Columbia Cancer Agency in Vancouver said the study showed "real but modest" benefits for screening and said the new analysis "reassures us that the Swedish data are believable."

Proponents of mammography rejected that characterization of the benefits as modest.

"I wouldn't use that word," said Dr. Peter Greenwald, director of cancer prevention at the National Cancer Institute. "I think when you get a reduction of one-fifth in deaths from breast cancer, that's very important."