In clinical trials, screening mammography has been shown to reduce mortality from breast cancer by about 25% to 30% among women aged 50 years and older after only 5 to 6 years from the initiation of screening. Among women 40 to 49 years old, the evidence supporting the efficacy of screening mammography is less convincing. Eight randomized controlled trials have reported no statistically significant reduction in breast cancer mortality among women age 40 to 49 years after 7 to 9 years from the initiation of screening. After 10 to 12 years from the initiation of screening, there is a trend toward a reduction in breast cancer mortality that appears to be smaller than that observed in women age 50 and older. Even if there is a small delayed reduction in breast cancer mortality among women age 40 to 49 years, the potential absolute benefit of screening women in this age group is likely to be small, given that the incidence of breast cancer is low. Since there is strong scientific evidence that mammography reduces breast cancer mortality among women age 50 and older, our national focus should be on screening these women who have the most to benefit.