Relationships between menstrual factors and breast cancer risk were investigated in a prospective study of 63,090 Norwegian women. A total of 1565 cases of breast cancer occurred during follow-up from 1961 through 1980. The risk of breast cancer decreased with increasing age at menarche (P = 0.06) and increased with increasing age at menopause (P = 0.005) in analyses adjusted for age, urban/rural place of residence, parity, and age at first and last birth. The results correspond to an average increase in breast cancer risk of 4.0% for each year of decrease in age at menarche, and an increase in risk of 3.6% for each year of increase in age at menopause. The protective effect of early menopause was strongest for breast cancer diagnosed in patients 80 years of age or older. No clear relationship was seen between menstrual irregularities and breast cancer risk.