According to a large study conducted in Norway, mammograms reduce the risk of dying from breast cancer by 10 percent.
Only a third of that reduction is due to the screening itself. The rest is because of better treatment and greater awareness of the disease.
According to these findings, 2,500 women would have to be regularly screened over a 10-year period to save one life.
The research is published in Thursday's New England Journal of Medicine.