Abstract： Background There is an association between postmenopausal tamoxifen therapy and endometrial pathologies. We investigated the usefulness of diagnostic hysteroscopy and transvaginal ultrasonography (TVS) and estimated whether diagnostic hysteroscopy improves detection of endometrial pathologies in postmenopausal breast cancer patients on tamoxifen.Methods Ninety-seven postmenopausal breast cancer patients who had been taking tamoxifen 20 mg/d for ≥6 months went through TVS, diagnostic hysteroscopy, and endometrial biopsy examinations. The presence of endometrial histopathologic features with abnormal TVS and diagnostic hysteroscopic findings were correlated.Results No endometrial cancer was found in any of the 97 patients. Fifty-three patients (54.6%) developed endometrial polyps as diagnosed histopathologically. Fifty-nine patients (60.8%) tested positive in TVS exams, of whom 43 had polyps, four had hyperplasia, and 12 atrophy. Thirty-eight patients (39.2%) tested negative in TVS exams, of whom 10had polyps, three hyperplasia, and 25 atrophy. TVS exams presented 63.6% specificity, 81.8% sensitivity, 72.9%positive-predictive value, and 73.7% negative-predictive value, whereas the corresponding values of diagnostic hysteroscopy were 100%, 98.1%, 100%, and 97.8% respectively. The correct ratio of hysteroscopy was significantly higher than that of TVS (P=0.000).Conclusions In postmenopausal breast cancer patients treated with tamoxifen, TVS alone is not sufficient for the detection of endometrial pathologies. Additional use of diagnostic hysteroscopy considerably improves the detection of polyps, thus significantly reducing the rate of false-negative findings of endometrial pathologies.