The aim of our study was to estimate the incidence of favorable mammographic patterns and to assess the incidence by risk factors of breast cancer. Data relate to 1,947 women aged 40-47 years who were screened by mammography every second year and whose mammographic parenchymal pattern at initial screening was either P2 or DY (Wolfe's classification). The pattern was assessed at each screening round. The incidence of N1,P1 parenchymal pattern by risk factors was estimated. The Cox proportional hazards model was applied in univariate and multivariate analyses of the data. The incidence of N1,P1 pattern was 12.5/100 woman years among women with P2,DY pattern at the beginning of follow-up. The relative risk of N1,P1 pattern adjusted for the other risk factors for women aged 45 years or more was 1.7 [95% confidence interval (CI) 1.5-1.9] compared with younger women. The adjusted relative risk of N1,P1 pattern among women with a body mass index (BMI) of 25 or more was 2.1 (95% CI 1.6-2.8) compared with women with a BMI of less than 20; women with more than 2 pregnancies had a 30% higher adjusted relative risk (RR = 1.3, 95% CI 1.1-1.6) than women with no pregnancies. The incidence of favorable mammographic parenchymal pattern was significantly related to old age, large BMI and multiparity. The effects of age and BMI on incidences of favorable and unfavorable change potentiate each other but not that of parity.