BACKGROUND The efficacy of mammographic screening in the reduction of breast carcinoma mortality has been demonstrated in randomized controlled trials. However, the evaluation of organized screening outside of research settings (so-called "service screening") faces unique methodologic and conceptual challenges. The current study describes the evaluation of organized mammography screening in a clinical setting and demonstrates the benefit obtained from service screening in two Swedish counties. METHODS In the group of subjects ages 20--69 years, there were 6807 women diagnosed with breast carcinoma over a 29-year period in 2 counties in Sweden and 1863 breast carcinoma deaths. All patients were classified from patient charts based on their screening status (i.e., whether they had been invited to undergo screening and whether they actually had undergone screening). The number of women who lived in the 2 counties during the 29-year study period was provided by the Central Bureau of Statistics. Breast carcinoma-specific mortality was compared across three time periods: 1) 1968--1977, when no screening was taking place because mammography had not been introduced; 2) 1978--1987, the approximate period of the Two-County randomized controlled trial of screening in women ages 40--74 years; and 3) 1988--1996, when all women in the 2 counties ages 40--69 years were invited to undergo screening (service screening). When comparing breast carcinoma mortality in screened women with that in women diagnosed before screening was introduced, a correction for self-selection bias was incorporated to prevent overestimation of the benefit of screening. RESULTS The mortality from incident breast carcinoma diagnosed in women ages 40-69 years who actually were screened during the service screening period (1988--1996) declined significantly by 63% (relative risk [RR] = 0.37; 95% CI, 0.30--0.46) compared with breast carcinoma mortality during the time period when no screening was available (1968--1977). The mortality decline was 50% (RR = 0.50; 95% CI, 0.41--0.60) when breast carcinoma mortality among all women who were invited to undergo screening (nonattendees included) was compared with breast cancer mortality during the time period when no screening was available (1968--1977). The reduction in mortality observed during the service screening period, adjusted for selection bias, was 48% (RR = 0.52; 95% CI, 0.43--0.63). No significant change in breast carcinoma mortality was observed over the three time periods in women who did not undergo screening. This group included women ages 20--39 years because these individuals were never invited to undergo screening, and women ages 40--69 years who did not undergo screening (not invited during the randomized trial or invited during the second and third time periods but declined). CONCLUSIONS Regular mammographic screening resulted in a 63% reduction in breast carcinoma death among women who actually underwent screening. The policy of invitation to organized screening with mammography appears to have reduced breast carcinoma mortality by 50% in these 2 counties.