The relationship between sequential mammographic parenchymal patterns and breast cancer was estimated and the results were applied to selective screening. In a pilot screening program 4163 Finnish women aged 40-47 years at entry were invited to be screened every second year from 1982 to 1990. Mammographic parenchymal patterns (Wolfe's classification) were recorded at each screening round. The follow-up ended in 1993 and up until that time 68 new breast cancers were diagnosed. The age-adjusted relative risk of breast cancer was 2.5 (95% CI 1.5-4.0) among women with high-risk mammographic parenchymal patterns (P2,DY) at the screenings preceding cancer diagnosis compared with those with low-risk patterns (N1,P1). After further adjustment for body mass index, number of pregnancies and size of the breast, the relative risk increased to 2.8 (95% CI 1.7-4.9). The mammographic parenchymal pattern is an independent risk factor of breast cancer but not strong enough to be used as a criterion for selective screening.