Bilateral oophorectomy is often performed during hysterectomy for benign conditions and can reduce breast cancer risk by 20%-50% when performed at younger ages. Accuracy of estimating the decrease in breast cancer risk associated with bilateral oophorectomy could be affected by common conditions that lead to surgery, such as uterine fibroids or endometriosis. The authors examined the potential for confounding by nonmalignant indications for surgery on breast cancer risk estimates in a population-based case-control study of invasive breast cancer newly diagnosed in 1992-1995. Breast cancer cases (N = 4,935) aged 50-79 years were identified from Wisconsin, Massachusetts, and New Hampshire tumor registries; similarly aged controls (N = 5,111) were selected from driver's license and Medicare lists. Reproductive and medical history was obtained from structured telephone interviews. Odds ratios and 95% confidence intervals were estimated with multivariate logistic regression. Women who underwent bilateral oophorectomy with hysterectomy at age ≤ 40 years had significantly reduced odds of breast cancer (odds ratio = 0.74, 95% confidence interval: 0.60, 0.90) compared with women with intact ovaries and uterus. Effect estimates were virtually unchanged after adjustment for uterine fibroids or endometriosis history. Results indicate that breast cancer risk reductions conferred by bilateral oophorectomy are not strongly confounded by failure to account for nonmalignant indications for surgery.