Aim. Tamoxifen is the antihormonal treatment of choice for premenopausal breast cancer patients with advanced breast disease. Its premenopausal administration has been shown to induce supraphysiological 17β-estradiol serum levels and to be associated with the presence of persistent, bilateral functional ovarian cysts. However, these abnormalities have not yet been compared to controls. In this study we evaluated the possibility that the above hormonal and/or ovarian abnormalities are more frequent among premenopausal breast cancer patients treated with tamoxifen than among similar nontreated patients, and thus they may be attributed to tamoxifen effect. Methods. We evaluated serum hormone levels of 17β-estradiol, follicular-stimulating hormone, lutenizing hormone, and progesterone, the presence of ovarian cysts, and various demographic and clinical characteristics in 20 premenopausal breast cancer patients treated with tamoxifen (study group) and compared them to those observed in 12 similar nontreated patients (control group). Results. Ovarian cysts were found in 80% of the study patients and only in 8.3% of the control patients (P = 0.001). The incidence of oligomenorrhea was nearly significantly higher in the study than in the control group (50 and 16.7%, respectively; P = 0.0651). Various serum hormone levels tested were not found to be significantly different between the two groups, except for 17β-estradiol serum levels as detected on days 14 and 21 of the menstrual cycle, which were significantly higher among the study than in the control patients. (Day 14 serum estradiol: 757.7 ± 372.0 pg/mL versus 206.5 ± 275.0 pg/mL, P = 0.0012. Day 21 serum estradiol: 300.0 ± 134.5 pg/mL versus 96.5 ± 71.5 pg/mL, P = 0.0008.) Conclusions. Tamoxifen treatment increases the incidence of ovarian cysts and the significantly higher 17β-estradiol serum levels in premenopausal breast cancer patients.