They prevent cancer deaths, and the number of lives saved increases over time.
The researchers found that seven years of mammograms made for 30 percent fewer breast cancer deaths years down the road, when compared with women who didn't receive mammograms.
That translates to 15 to 20 thousand lives a year.
Past studies only looked 7 to 10 years down the road, so the benefit didn't seem as large.
And that's what the U.S. Preventive Services Task Force used when it recommended in 2009 that routine mammograms not start till age 50, and that they end at 75. It cited the large number of mammograms needed to save a life, and a higher risk of false positives that might lead to unneeded biopsies and other tests.
The new study of 130,000 Swedish women says that it only takes 414 to 519 screenings to save on life.
Some cancer experts say the benefits are probably even bigger now, because mammogram technology has improved so much since the study started.
The study in Sweden didn't specifically address the controversy over screenings for women between 40 and 49 years of age, because it didn't break down the risk according to age.
Despite the federal recommendations, the American Cancer Society continued calling for mammograms to start at 40.
