OBJECTIVE To evaluate sonographic measurements of endometrial thickness in postmenopausal women taking adjuvant tamoxifen therapy for breast cancer, and to correlate sonographic and pathologic findings to symptoms and duration of tamoxifen therapy. METHODS Medical records and sonograms of 80 postmenopausal women treated for breast cancer with adjuvant tamoxifen therapy were reviewed retrospectively. Endometrial thickness was recorded as a single-layer thickness and considered abnormal when greater than 2.5 mm for postmenopausal women. Sonographic endometrial thickness was correlated to histologic findings, symptoms, and duration of tamoxifen therapy. RESULTS Fifty-seven of 80 postmenopausal women (69%) had single-layer endometrial thicknesses of 2.5 mm or greater, measured by transvaginal sonography, and 55 of 57 had endometrial biopsies or dilatations and curettage. Biopsies detected 24 cases of abnormal endometria, including endometrial carcinoma (two), breast carcinoma metastatic to the endometrium (one), endometrial polyps (13), tubal metaplasia (three), and benign endometrial hyperplasia (five). Using a single-layer endometrial thickness greater than 2.5 mm by transvaginal ultrasound, 21 of 24 (87.5%) women with abnormal endometria were detected. Women with abnormal pathologic findings had a significantly thicker mean single-layer endometrial thickness than those with normal findings, 7 mm versus 4 mm (P < .01). Twelve women had postmenopausal bleeding, all of whom had a single-layer endometrial thickness greater than 2.5 mm on transvaginal sonography. CONCLUSIONS With a sensitivity of detecting endometrial abnormalities of 84%, transvaginal sonography was useful for studying postmenopausal tamoxifen-treated women.