PURPOSE: Tamoxifen is a non-steroidal antiestrogenic drug used mainly in the adjuvant therapy of breast cancer and it also has estrogenic effect to the endometrium. We investigated the effects of tamoxifen on endometrial thickening in postmenopausal women taking adjuvant tamoxifen therapy for breast cancer, and analyzed the correlation between sonographic findings, pathologic findings and duration of tamoxifen therapy. MATERIALS AND METHODS: Forty three patients previously operated and being treated by tamoxifen since July 1995 to August 1999 were involved in this study. Control group was selected from patients of postmenopausal syndrome visiting postmenopausal clinic more than 2 years after menopause since January 1994 to August 1997. Endometrial thicknesses of breast cancer patients taking tamoxifen were measured twice a year for 2 years and then once a year by transvaginal ultrasonography. They were considered abnormal when those thicknesses were greater than 6 mm. Sonographic mearsurements of control patients were done once at the first visit and which were compared with those of breast cancer patients. RESULTS: The mean endometrial thicknesses of breast cancer patients receiving tamoxifen therapy were 5.06 mm (4.86~5.21 mm) at 6th month, 5.1 mm (4.92~6.00 mm) at 12th month, 5.13 mm (4.89~5.32 mm) at 18th month, 5.15 mm (4.97~5.28 mm) at 24th month and 5.14 mm (4.96~5.21 mm) at 36th month. The mean thickness of control patients was 4.87 mm. The mean endometrial thickness of breast cancer patients and that of control patients were 5.07 mm and 4.87 mm respectively. Only one patient with stage I breast cancer showed endometrial thicknessof 6 mm during follow-up and endometrial biopsy showed atypical endometrial hyperplasia. Endometrial thickness significantly increased after 18 months of tamoxifen therapy, but the rate of increase was slow. CONCLUSION: The endometrial thickness increased with duration of tamoxifen therapy, but the rate of increase was slow and seldom exceeded 6 mm. So we concluded the risk of endometrial cancer is low.