Forty-six nonhysterectomized women treated with tamoxifen during 6-36 months as adjuvant therapy for breast cancer underwent a hysteroscopy to assess the endometrial effects of this drug. Whereas the endometrium was normal among 23 patients, 13 presented with endometrial polyps, 8 with hyperplasia and 2 with adenocarcinoma. The rate of endometrial lesions was directly related to the cumulative dose of tamoxifen but it was not statistically different among patients receiving progestational therapy compared to patients who did not receive this therapy.