A cohort study of the occurrence of breast cancer among 6,908 women undergoing artificial menopause from 1920 through 1940 and among 1,479 women having natural menopause is presented. The nature of the artificial menopause was determined from surgical and pathologic records in each case, and follow-up information through 1961 was obtained from hospital records, death certificates, and Tumor Registry data. The expected mortality from, and incidence of, breast cancer among the study cohorts were calculated from published rates and adjusted for estimated migration and incompleteness of follow-up. Among 1,278 women with artificial menopause induced by hysterectomy with bilateral oophorectomy before age 40, six had breast cancer, 3 of whom died, as compared to an expected incidence of 24.0 patients with 11.3 deaths. The differences in both the incidence and mortality rates were highly significant. Incidence of, and mortality from, breast cancer in the cohorts of women undergoing noncastrating gynecologic surgery or receiving pelvic irradiation before age 40 did not differ significantly from expected values or from the rates among a control group of women undergoing cholecystectomy. Breast cancer rates among women undergoing gynecologic surgery after age 40, with or without castration, also did not differ significantly from expected values or from the rates among appropriate controls, but the incidence of breast cancer among women age 40–44, receiving pelvic irradiation, was significantly lower than expected. The protective effect of early surgical castration was observed in both parous and nulliparous women. It is concluded that, by curtailing cyclic ovarian function, surgical castration before age 40 reduces the risk of developing breast cancer by approximately 75%, but that other forms of artificial menopause or castration after age 40 does not afford such protection. Pelvic irradiation may also have a protective effect, but the relationship to age is not entirely clear.