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Objectives: To evaluate the effectiveness of mammography service screening., Setting: Helsinki (population 0.5 million), the capital of Finland, the breast cancer service screening programme was started gradually in 1986., Materials and Methods: The study was based on the data from the breast cancer screening programme in Helsinki. All incident cases of breast cancer and rates in the female population in Helsinki during the period 1970-97, and information on the subsequent breast cancer mortality were derived from the files of the Finnish Cancer Registry. To estimate the sensitivity of the programme, the number of screen detected cancers were compared with the overall number of breast cancers at the targeted age groups. Effects of screening were estimated for cumulative incidence and for the refined mortalities among the birth cohorts screened., Results: Altogether 144 400 invitations had been sent during 1986-97 among the 50-9 year old women as targeted in Helsinki. The average attendance rate was 82%; and 545 breast cancers were detected by screening, with a detection rate of 0.46%. In the birth cohort of women born between the start of 1935 and the end of 1939-namely, those subjected to most complete screening and long follow up time since the onset of the programme-there was an increase in the cumulative incidence of breast cancer (relative risk (RR) 1.18, 95% confidence interval (95% CI) 1.09 to 1.27). The estimated sensitivity of the screening programme was 58%; and 53% if corrected for overdiagnosis. There was also a decrease of 19% (RR 0.81, 95% CI 0.62 to 1.05) in the refined mortality within this screened birth cohort, compared with the death rate of women of comparable ages in the non-screened reference cohort., Conclusion: The study provides further support to the hypothesis that service screening with mammography reduces the risk of breast cancer mortality.