ASHINGTON, Feb. 21 — After months of controversy over whether mammograms save lives, federal health officials today issued new guidelines for breast cancer screening that strongly recommend the tests and lower the age at which women should begin receiving them to 40 from 50.

The guidelines, announced by Tommy G. Thompson, the secretary of health and human services, stem from an analysis by an independent committee of experts that advises the health agency on preventive medicine. The guidelines are the federal government's official policy on mammograms.

"Mammography is not a perfect tool," Mr. Thompson said, acknowledging that recent studies have raised "legitimate issues" about breast cancer screening. But, he added, "Mammography is an important and effective early detection tool that helps to save lives."

Annual mammograms have long been routine for millions of American women, but the question of whether they truly reduce breast cancer deaths has been debated for years. The controversy was renewed last October, when a group of scientists in Denmark said they had found serious flaws in seven large studies that doctors believed had proved the benefits of mammography.

The Danish study prompted a group of American cancer experts to conclude in January that the benefits of breast cancer screening did not necessarily outweigh the risks, which include unnecessary surgery to remove tumors that might never have caused a problem. But after a review of the same studies that began in 1999, the preventive medicine committee reached a very different conclusion.

"Our bottom line," said Dr. Janet Allan, vice chairwoman of the committee, "is that mammograms reduce deaths from breast cancer."

The committee's recommendation, that women ages 40 to 69 undergo mammograms every one to two years, rests on its willingness to accept a number of studies that the group in Denmark discounted. While acknowledging that these studies had flaws and that "the flaws were problematic," the committee said the problems were not severe enough to dismiss the work.

Over all, Dr. Allan said, the committee concluded that women who received mammograms routinely were 23 percent less likely to die from breast cancer than women who did not. Thus the committee said it had found "fair evidence" that screening every 12 to 33 months "significantly reduces mortality from breast cancer."

The 15-member committee, called the United States Preventive Services Task Force, published two earlier breast cancer screening recommendations, in 1989 and 1996. Both endorsed screening for women ages 50 to 69, and today the panel said the evidence in favor of mammography was strongest for that age group.

Because the risk of breast cancer is lower in women under 50, the benefits of regular mammography are smaller for this age group. But evidence is emerging from continuing studies that younger women also benefit, Dr. Allan said. While the committee's report said it was "difficult to determine the incremental benefit of beginning screening at age 40 rather than age 50," the panel nonetheless recommended that women do so.

That Mr. Thompson announced the panel's recommendations is unusual. Typically, such reports are published in a medical journal. Advocates for breast cancer patients said they expected the committee's findings to be published in April, in The Annals of Internal Medicine.

Mr. Thompson, whose wife was successfully treated for breast cancer seven years ago after a tumor was identified by a mammogram, made clear today that he felt a responsibility to speak out.

"Women are confused as to whether they should have a mammogram or not," Mr. Thompson said. "I feel that, as secretary of health and as someone who has personally been through this, it was the right thing to do."

Breast cancer is the most common cancer among women in the United States and, after lung cancer, the second-leading cause of cancer-related death, according to the health and human services agency. In 2001, the department said, the cancer was diagnosed in 192,200 women, and 40,600 died from the disease.

Today's recommendations, and Mr. Thompson's endorsement of them, were greeted warmly by advocates for breast cancer patients and doctors who care for them, who have worried that a recent drop in breast cancer deaths will turn around if women stop seeking mammograms.

Some feared that the controversy would result in financing cuts to programs that promote mammography and would prompt insurance companies to stop covering the tests. Today's recommendations are likely to prevent that from happening, advocates said.

"I'm very happy," said Dr. Carolyn Runowicz, a gynecologist-oncologist and breast cancer survivor who serves on the board of the American Cancer Society.

"It's a very well-respected group," Dr. Runowicz said, referring to the committee. "They examine data very carefully and they come up with what they feel is good evidence- based medicine, and they are politically independent."

Zora Brown, founder of the Breast Cancer Resource Committee, an advocacy group that seeks to lower breast cancer rates among African- American women, said, "Hopefully this will put to rest some of the controversy."

Others predicted the controversy would only grow. Among them was Dr. Donald Berry, chairman of the department of biostatistics at M. D. Anderson Cancer Center in Houston and a member of the so-called P.D.Q. panel of cancer experts that in January cast doubt on the validity of mammograms.

"They pooh-pooh some of the criticism of the trials, and I'd like to know on what grounds," said Dr. Berry, who has emerged as an outspoken critic of mammography.

The idea behind mammograms is that there is a benefit to detecting tumors early on, when they are smaller. But there are downsides, the federal committee found. The large majority of abnormal mammograms, 80 percent to 90 percent, are false positives, which can prompt a woman to undergo invasive procedures, like breast biopsy, that may be unnecessary.

The P.D.Q. board, which does not make policy recommendations but writes information for the National Cancer Institute, highlighted these drawbacks in January. The panel said that it was rational for women to decide to have mammograms and that it was also rational for them to decide not to.

Today, Dr. Berry said the panel would write up its findings, despite the new federal guidelines. At the same time, however, officials at the National Cancer Institute affirmed their support for mammography, even as they spoke of the need to "seek improved methods of diagnosis and treatment of breast cancer."

In addition to addressing mammography, the panel examined the benefits of self-examinations and clinical breast exams performed by doctors. It said that while these techniques detected some additional cancers there was not enough data to determine whether they reduced deaths from breast cancer.