The association between mammographic parenchymal patterns and the risk of subsequent breast cancer was studied in a cohort of 17,911 women aged 40-70 years, enrolled in a mammographic screening program. The cohort was prospectively followed-up for 5 years after blind attribution of mammographic pattern. Incident cancers, either screen- or interval-detected, were monitored through the screening program and by the local cancer registry. Multivariate analysis adjusting for the confounding effect of age, demonstrated an independent and significant association between mammographic patterns and breast cancer risk. The relative risk of subsequent breast cancer, assumed to be 1 for N1 pattern, was 2.39 (1.0-5.7) for P1, 4.02 (1.7-9.3) for P2, and 5.01 (1.9-13.3) for DY category, respectively. The relative risk of P2-DY with respect to N1-P1 categories was 2.09 (1.4-3.1). A diagnostic delay, possibly due to the 'masking effect' of a radiologically dense breast, was evident for P2-DY with respect to N1-P1 cancers (proportion of Stage I cancers = 42% vs. 69%). These results suggest the adoption of a shorter rescreening interval in P2-DY subjects.