OBJECTIVE To determine the prevalence of endometrial thickening and endometrial pathologies in postmenopausal breast cancer patients taking tamoxifen. METHODS A total of 37 postmenopausal breast cancer patients receiving 20 microg/day of tamoxifen treatment for at least 6 months at Srinagarind hospital were included in the study. Thorough history taking and physical examination as well as transvaginal ultrasonography were conducted for all patients. Fractional curettage was carried out in those whose endometrial thickness was found to be greater than 5 mm. RESULTS Among 37 patients included in this study, the mean age was 56.35 years. The mean body weight and mean body mass index was 60.88 kg and 26.03 kg/m(2), respectively. The majority of patients (75.68%) had stage II disease. The mean + SD of endometrial thickness found in this study was 7.53 + 5.16 mm. The prevalence of thickened endometrium (defined as ET > 5mm from TVS) was 59.46%. Among the 19 patients for whom fractional curettage was conducted, the majority (73.69%) exhibited inadequate endometrium for evaluation. Atrophic endometrium and other unremarkable changes were found in 21.05% of patients and it is important to note that endometrial adenocarcinoma was detected in 1 case (5.26%). CONCLUSIONS The prevalence of thickened endometrium in postmenopausal breast cancer patients taking tamoxifen found in this study was extraordinarily high. These is, however, a discrepancy between the value and that for endometrial abnormalities detected histologically.