Objective: To investigate the effects of tamoxifen on the endometrium in postmenopausal breast cancer patients. Methods: Endometrial thickness was measured by transvaginal sonography and endometrial biopsies were done in 104 postme­nopausal breast cancer cases who were treated with tamoxifen. Histopathologic findings were discussed. Results: Mean endometrial thickness was 11.7 ± 5.9 mm and duration of tamoxifen administration was 35.3 months. Four endometrial cancers, 17 endometrial hyperplasias, 25 proliferative endometrium, 5 endometrial polyps in the endometrial biopsies. We observed atrophic endometrium in 53 of the cases. Only one case with endometrial polyps was observed as a premalignant lesion when the endometrium was less than 5 mm, 51 % of the cases had thicker endometrium (more than 10 mm) and 32% of these cases had malignant and premalignant endometrium. We found a significant correlation with the duration of tamoxifen and age (p < 0.05) One hundred and two of our cases were asymptomatic; only 2 out of 4 endometrial cancer cases had vaginal spotting. A significant relation was noticed between endometrial thickness and duration of tamoxifen treatment (p = 0.025). Conclusion: It was concluded that positive endometrial findings and endometrial thickness were due to continuous unopposed tamoxifen treatment and our findings support the hypothesis that tamoxifen increases the risk of endometrial carcinoma and pre­malignant changes.