CONTEXT: Women with BRCA1 or BRCA2 mutation are often advised to undergo preventive oophorectomy. The effectiveness of this intervention has not been prospectively evaluated in a large cohort. OBJECTIVES: To estimate the incidence of ovarian, fallopian tube, and primary peritoneal cancer in women who carry a deleterious mutation in BRCA1 or BRCA2. To estimate the reduction in risk of these cancers associated with a bilateral prophylactic salpingo-oophorectomy. DESIGN, SETTING, AND PARTICIPANTS: Women known to carry a BRCA1 or BRCA2 mutation were identified from an international registry between 1992 and 2003. A total of 1828 carriers at 1 of 32 centers in Canada, the United States, Europe, and Israel completed questionnaires at baseline and follow-up. Participants were observed from the date of study entry until: diagnosis of ovarian, fallopian tube, or peritoneal cancer; death; or the date of the most recent follow-up. INTERVENTION: Participants were divided into women who had undergone bilateral prophylactic oophorectomy and those who had not. MAIN OUTCOME MEASURE: The incidence of ovarian, peritoneal, and fallopian tube cancer was determined by survival analysis. The risk reduction associated with prophylactic salpingo-oophorectomy was evaluated by a time-dependent survival analysis, adjusting for covariates. RESULTS: After a mean follow-up of 3.5 years, 50 incident ovarian, fallopian tube, and peritoneal cancer cases were reported in the cohort. Of the 1828 women, 555 (30%) underwent a bilateral prophylactic salpingo-oophorectomy prior to study entry, 490 (27%) underwent the procedure after entering the study, and 783 (43%) did not undergo the procedure. There were 32 incident cancers diagnosed in women with intact ovaries (1015/100,000 per year). Eleven cancer cases were identified at the time of prophylactic oophorectomy and 7 were diagnosed following prophylactic oophorectomy (217/100,000 per year). The estimated cumulative incidence of peritoneal cancer is 4.3% at 20 years after oophorectomy. The overall (adjusted) reduction in cancer risk associated with bilateral oophorectomy is 80% (multivariate hazard ratio = 0.20; 95% confidence interval, 0.07-0.58; P = .003). CONCLUSION: Oophorectomy is associated with reduced risk of ovarian and fallopian tube cancer in high-risk women, although there is a substantial residual risk for peritoneal cancer in BRCA1 and BRCA2 mutation carriers following prophylactic salpingo-oophorectomy.