The effect of age at first pregnancy and number of pregnancies was examined in a cohort of 14,421 women where reproductive informations were collected as part of a cervical cancer screening program. A total of 352 cases occurred in the cohort. In subgroups of women defined by age at first pregnancy the relative risk of breast cancer was lower among women who were first pregnant before 25 years of age (RR = 0.85 vs 1.20). In subgroups of women defined by parity the relative risk decreased with increasing number of pregnancies from about unity among women with 0-3 pregnancies to 0.7 among women with 6 or more pregnancies.