The authors investigated the relations of demographic, anthropometric, and lifestyle factors with endometriosis in the Nurses' Health Study II prospective cohort. During 10 years of follow-up (1989-1999), 1,721 cases of laparoscopically confirmed endometriosis were reported among women with no past infertility. The incidence rate was greatest among women aged 25-29 years and lowest among women over 44 years (p(trend) < 0.0001). In multivariate Cox proportional hazards models, African-American women had a lower rate of disease compared with Caucasian women (rate ratio = 0.6, 95% confidence interval: 0.4, 0.9). The authors also observed an inverse relation with body mass index at age 18 years (for body mass index of >30 vs. 19-20.4 kg/m(2): rate ratio = 0.8, 95% confidence interval: 0.6, 1.1; p(trend) = 0.004) and with current alcohol intake (for >10 vs. 0 g/day: rate ratio = 0.7, 95% confidence interval: 0.6, 0.8; p(trend) < 0.0001) but no association with height, waist/hip ratio, or caffeine intake. An inverse relation with current body mass index and current cigarette smoking was observed only when cases were concurrently infertile. The authors conclude that age, race, body mass index, alcohol use, and cigarette smoking are associated with the incidence of endometriosis and that some of these relations may differ by infertility status at the time of laparoscopic diagnosis.