OBJECTIVE Our purpose was to investigate the frequency of ovarian cysts in tamoxifen-treated breast cancer patients. METHODS The study population included 95 consecutive tamoxifen-treated premenopausal and postmenopausal women with breast cancer who were followed up by the outpatient clinic at the Hadassah University Hospital between September 1990 and June 1992. Tamoxifen was administered orally (20 mg/day). All patients underwent a pelvic examination and vaginal ultrasonography with a 5 MHz vaginal probe. RESULTS During the study 11 of 95 tamoxifen-treated breast cancer patients (11%) had ovarian cysts. Five cysts were detected in postmenopausal women (6.3% of the postmenopausal women) and six in premenopausal women (37.5% of the premenopausal women). In postmenopausal and premenopausal women the mean tamoxifen treatment interval was 19.4 +/- 7.8 months (range 4 to 48 months) and 28 +/- 6.1 months (range 12 to 54 months), respectively (p = 0.41). In 8 of the 11 patients the ovarian cystic enlargement disappeared after cessation of tamoxifen treatment. Two patients underwent laparotomy because of persistent cysts and the third because of a rapidly growing myoma. The three cysts were found to be benign. CONCLUSIONS Ovarian cysts are a common side effect of tamoxifen treatment. The ovarian cysts can develop in tamoxifen-treated premenopausal as well as postmenopausal women with breast cancer. Most of the tamoxifen-associated cysts disappear after tamoxifen treatment is abandoned.