The role of age at menarche and at menopause on breast cancer risk was reassessed in a combined analysis of four Italian case-control studies including a total of 6,075 cases and 5,492 controls. The risk of breast cancer was lower in women whose menarche occurred at age 15 or over, but there was no evidence for the risk to increase with decreasing age at menarche below age 15. Compared with women with earlier menarche, the relative risk (RR) was 0.9 (95% confidence interval, CI 0.7-1.0) for those with menarche at age 15, 0.8 (95% CI 0.6-0.9) for menarche at 16, and 0.7 (95% CI 0.5-0.8) for menarche at age 17 or over. There was no significant interaction between age at menarche and study centre or age at diagnosis, parity and age at first birth. In relation to age at menopause, compared with women whose menopause occurred at age 40 or less, the relative risk was 1.1 (95% CI 0.8-1.3) between 40 and 44, 1.2 (95% CI 0.9-1.4) between 45 and 49, 1.4 (95% CI 1.2-1.8) between 50 and 53, and 1.4 (95% CI 1.1-1.8) above 53. The risk estimates were comparable in various studies, and the trend in risk with age at menopause was statistically significant. The risk estimates tended to be somewhat higher at peri-menopausal age (45 to 54 years), but no consistent pattern was evident across subsequent strata of age, and the interaction with age was not significant. Likewise, no consistent interaction was observed with parity, age at first birth or body mass index.(ABSTRACT TRUNCATED AT 250 WORDS)