Background: Women with BRCA1 or BRCA2 mutations have an increased risk of breast cancer and ovarian cancer. Prophylactic mastectomy and oophorectomy are often considered as ways of reducing these risks, but the effect of the procedures on life expectancy has not been established. Methods: In a decision analysis, we compared prophylactic mastectomy and prophylactic oophorectomy with no prophylactic surgery among women who carry mutations in the BRCA1 or BRCA2 gene. We used available data about the incidence of cancer, the prognosis for women with cancer, and the efficacy of prophylactic mastectomy and oophorectomy in preventing breast and ovarian cancer to estimate the effects of these interventions on life expectancy among women with different levels of risk of cancer. Results: We calculated that, on average, 30-year-old women who carry BRCA1 or BRCA2 mutations gain from 2.9 to 5.3 years of life expectancy from prophylactic mastectomy and from 0.3 to 1.7 years of life expectancy from prophylactic oophorectomy, depending on their cumulative risk of cancer. Gains in life expectancy decline with age at the time of prophylactic surgery and are minimal for 60-year-old women. Among 30-year-old women, oophorectomy may be delayed 10 years with little loss of life expectancy. Conclusions: On the basis of a range of estimates of the incidence of cancer, prognosis, and efficacy of prophylactic surgery, our model suggests that prophylactic mastectomy provides substantial gains in life expectancy and prophylactic oophorectomy more limited gains for young women with BRCA1 or BRCA2 mutations.