Because of the temporal relations between reproductive risk factors and incidence of breast cancer, the authors developed a nonlinear Poisson regression that accounts for time and summarizes risk to age 70 years. Reproductive risk factors, benign breast disease, use of postmenopausal hormones, weight, and alcohol intake were evaluated as risk factors. Among 58,520 women aged 30–55 years in 1980, followed through June 1, 1994, 1,761 incident invasive breast cancer cases were identified. All risks are multivariate adjusted. History of benign breast disease is associated with a 57% increase (95% confidence interval (CI): 43%, 73%) in cumulative risk of breast cancer by age 70 years. Use of unopposed postmenopausal estrogen from ages 50–60 years increases risk of breast cancer to age 70 by 23% (95% CI: 6%, 42%) compared with a woman who never uses hormones. Ten years of use of estrogen plus progestin increases risk to age 70 years by 67% (95% CI: 18%, 136%). Compared with never drinking alcohol, one drink per day from age 18 years increases risk to age 70 by 7% (95% CI: 0%, 13%). Use of unopposed postmenopausal hormones for 10 years significantly increases the risk of breast cancer, and the addition of progestin further increases the risk. Am J Epidemiol 2000;152:950–64.