Sensitivity and specificity of first screen mammography in a randomized screening trial at five centers are reported. A total of 23,101 women underwent mammography; in 139, breast cancer was detected at first screening; in 20, less than 12 months after first screening; and in 47, at second screening. All 206 cancer cases were histologically confirmed, and 174 were defined as being detectable at first screening. Average length of follow-up for all women was 3.2 years. Interpretations of first screen mammograms by the center radiologists were matched to known outcomes. Simultaneous blind review was performed by a single reference radiologist with mammograms from all 206 cancer cases and those of a random sample of 739 women not known to have breast cancer at 15 months or more after initial screening. Overall, the five screening centers achieved a sensitivity of 69% (range, 60%-78%), a specificity of 94% (range, 93%-96%), a positive predictive value of 8.6% (range, 3%-16%), and a negative predictive value of 99.7% (range, 99.6%-99.9%).