SummaryIt is well established that Tamoxifen is an excellent adjuvant therapy in cases of breast cancer, with very few side effects. After menopause this drug can act as a weak estrogen at the level of the genital tract and cases of endometrial hyperplasia and adenocarcinomas have been reported. During premenopausal years its action depends on the mode of administra-tionxontinue or cyclical, during proliferative or luteal phase. The occurrence of ovarian cysts seems more frequent than in a control group. It should thus be advised to consider Tamoxifen as a good antioestrogen at the level of the breast but as an active hormonal substance at the gynaecological level, and to offer the patient the adequate follow-up which seems necessary.