Its new guidelines, which are different from those of some professional and advocacy organizations, are published online in The Annals of Internal Medicine. They are likely to touch off yet another round of controversy over the benefits of screening for breast cancer.

Dr. Diana Petitti, vice chairwoman of the task force and a professor of biomedical informatics at Arizona State University, said the guidelines were based on new data and analyses and were aimed at reducing the potential harm from overscreening.

While many women do not think a screening test can be harmful, medical experts say the risks are real. A test can trigger unnecessary further tests, like biopsies, that can create extreme anxiety. And mammograms can find cancers that grow so slowly that they never would be noticed in a woman’s lifetime, resulting in unnecessary treatment.

Over all, the report says, the modest benefit of mammograms  reducing the breast cancer death rate by 15 percent  must be weighed against the harms. And those harms loom larger for women in their 40s, who are 60 percent more likely to experience them than women 50 and older but are less likely to have breast cancer, skewing the risk-benefit equation. The task force concluded that one cancer death is prevented for every 1,904 women age 40 to 49 who are screened for 10 years, compared with one death for every 1,339 women age 50 to 59, and one death for every 377 women age 60 to 69.

The guidelines are not meant for women at increased risk for breast cancer because they have a gene mutation that makes the cancer more likely or because they had extensive chest radiation. The task force said there was not enough information to know whether those women would be helped by more frequent mammograms or by having the test in their 40s. Other experts said women with close relatives with breast cancer were also at high risk.