It suggests that the breast cancer screening test can lower the risk of dying of the disease by as much as 26 percent.
The benefit is much greater than what's been seen in earlier research.
Other studies prompted a U.S. science panel to recommend against routine screening for women in their 40s. The panel says women under 50 should discuss their individual situations with their doctors.
The study compared breast cancer deaths in areas of Sweden that offered mammograms to younger women and areas that did not. Results are in the journal Cancer.