Menstrual cycle characteristics and ovulatory infertility were evaluated in relation to breast cancer risk among 116,678 women in the Nurses' Health Study II, a prospective cohort study of female registered nurses who were aged 25–42 years and living in 14 US states at enrollment in 1989. During 396,299 person-years of follow-up between return of the baseline questionnaire and June 1993, 251 cases of breast cancer were identified in this cohort. The multivariate relative risk (RR) associated with age at menarche >13 years compared with age ≤12 years was 0.66 (95% confidence interval (CI) 0.44–0.99). Short and long menstrual cycle lengths at ages 18–22 years were associated with reduced risk. Compared with menstrual cycle length 26–31 days, the multivariate relative risks (95% CIs) for more extreme cycle lengths were: <26 days, 0.50 (0.25–0.98); 32–39 days, 0.81 (0.51–1.28); and >39 days or too irregular for estimation of a usual cycle length, 0.41 (0.18–0.94). The multivariate relative risk associated with a history of ovulatory infertility, compared with no such history, was 0.41 (95% CI 0.18–0.93). These results are consistent with the hypothesis that reduced exposure to ovulatory menstrual cycles provides a protective effect against breast cancer. Am J Epidemiol 1998; 147: 636–43.