The purpose of the study was to estimate the incidence of unfavourable mammographic pattern by risk factors of breast cancers. Data consisted of 1890 Finnish women with mammographic pattern of either N1 or P1 (Wolfe's classification) at the initial screening. The screening was repeated every second year from 1982 to 1990 and at each screening round the mammographic pattern was assessed. The incidence rate of P2,DY pattern was 1.9/100 woman years. The incidence of P2,DY pattern was significantly related to age. The age-adjusted odds ratio (OR) (based on logistic regression) was 2.0 (95% CI 1.0-3.9) among women with hormonal replacement therapy (HRT), 0.6 (95% CI 0.4-0.9) among postmenopausal women, 0.2 (95% CI 0.1-0.4) among women with large breasts and 0.2 (95% CI 0.1-0.3) among women with large body mass index (BMI). After multivariate adjustment by logistic regression only the effect of BMI remained statistically significant, odds ratio of P2,DY pattern for women with BMI 25 or more was 0.2 (95% CI 0.1-0.6) compared to women with BMI less than 20.