STUDY OBJECTIVE: To assess the performance of breast cancer screening in different age categories over two decades. DESIGN: Important determinants of reduced breast cancer mortality such as attendance, mammography performance, cancer detection, and disease stage were recorded. SETTING: Nijmegen, The Netherlands, 1975-92. SUBJECTS: Since 1975 more than 40,000 women aged 35 years and older have been invited biennially for breast screening in a population based project in Nijmegen. MAIN RESULTS: Rates of attendance, referral, detection, and disease stage were calculated, as well as the specificity of screening mammography and the predictive value of referral and biopsy. From round 3 onwards, the attendance rate of women younger than 50 years stabilised at 70%, in women of 50-69 years it was 62%, and in women aged 70 and over it was 22%. In these three age categories, the referral rates of a positive screening mammography per 1000 screened women were 4.9, 6.2, and 11.8, respectively. Specificity rates were between 99% and 100%. Current predictive values of referral were high: in the specific age categories 39%, 59%, and 68% of the referred women had cancer. Detection rates remained fairly stable over the rounds 4-9, at 1.9, 3.6, and 8.0 cancers per 1000 screened women. In the two year period between screening the numbers of interval cancers per 1000 screened women were 2.2, 2.2, and 2.9, for the three age categories respectively. With regard to invasive cancers detected during screening, the percentage of small tumours (< or = 20 mm on the mammogram) was 84% in each age category. For women younger than 50 years, the proportion of intraductal carcinoma in all the cancers detected at screening was 40%, while it was 15% in the other age categories. CONCLUSION: Throughout the nine rounds, the screening outcomes were found to be adequate, particularly considering the high specificity rate and the predictive value of referral without the interference of a low detection rate. Although the occurrence of interval cancers seemed high, it was similar to other screening programmes. Despite a relatively low referral rate, the ratios of screen detected versus interval cancer cases were favourable. Well organised screening programmes can achieve good mammography results without too many false positives. It is important that women continue to participate in a screening programme because cancer can still be detected even after several successive negative screening examinations.