Clinical studies have reported associations between ovarian stromal hyperplasia and the diagnosis of hormonally related tumors such as endometrial cancer. To assess the hypothesis that characteristics of benign ovaries among postmenopausal women are related to risk for breast, endometrial, and colon cancer, we analyzed systematically collected transvaginal ultrasound data for participants enrolled in the screening arm of the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial. Among women without cancer, median ovarian volume declined with age from 1.25 cm3 for women between ages 55 and 59 years to 1.0 cm3 for those between ages 65 and 69 years. African American and Caucasian women had larger median ovarian volumes than Asians. Larger ovarian volume was also associated with the highest quartiles of height and weight and ever having smoked. After adjusting for race, age, parity, body mass, smoking, and hormone use, women with median ovarian volumes >or=3.0 cm3 were at increased risk for breast cancer [odds ratio (OR), 1.42; 95% confidence interval (95% CI), 1.11-1.70], endometrial cancer (OR, 1.97; 95% CI, 1.12-3.48), and colon cancer (OR, 2.00; 95% CI, 1.25-3.21). Significant trends of risk with increasing volume were found only for breast and endometrial cancers. We conclude that large ovaries among postmenopausal women may represent a marker of risk for hormonally related tumors. Confirmation of these findings in future studies, including analyses of serum hormone levels and tissues, may provide insights into hormonal carcinogenesis among older women.