From September 1, 1989, to November 30, 1994, 175 menopausal breast cancer patients treated with tamoxifen were followed at the authors' institutions. During this period, 16 (9.1%) underwent total abdominal hysterectomy and bilateral salpingo-oophorectomy, for various indications. Of these, 10 (62.5%) had either uni- or bilateral ovarian tumors. The analysis of surgical findings showed an incidence of 5.7% (10/175) ovarian tumors among all the patients. In 2 (20%), the ovarian masses displayed enlargement over a relatively short period while on treatment. In 5 (50%) patients, the findings were bilateral. All tumors were detectable by ultrasonography, except four serous cystadenomas found in 3 women. The mean duration of tamoxifen treatment was 36.6 ± 24.9 (range 9-86) months. The rate of 5.7% for ovarian tumors, in this selected group of patients, is four to five times higher than that reported for similar pathologic conditions detected by general screenings with ultrasonographic scans among nonselected, asymptomatic, and untreated postmenopausal women. Two possibilities should be considered in the development of ovarian tumors coinciding with tamoxifen treatment; (1) women with breast malignancy are prone to develop benign or malignant ovarian tumors in relation to genetic factors, regardless of tamoxifen treatment; and (2) tamoxifen may stimulate enlargement of such tumors and may even cause them.