OBJECTIVE. This study was performed to evaluate sonohysterography for the diagnosis of endometrial abnormalities in women treated with tamoxifen. MATERIALS AND METHODS. Fifty sonohysterograms were obtained in 48 consecutive tamoxifen-treated women. All women were postmenopausal and had been undergoing tamoxifen therapy for a mean of 2.6 years. Forty-six sonohysterograms (92%) were completed and four were unsuccessful. Sonohysterogram findings were correlated with prior endometrial biopsy results for 23 sonohysterograms (46%) that were preceded by endometrial biopsy. Sonohysterogram findings were also compared with histopathology results, available for 38 sonohysterograms (76%) that were followed by hysteroscopy with dilatation and curettage. RESULTS. Sonohysterography revealed 31 endometrial polyps (62%), six thickened endometria (12%), five normal endometria (10%), and four subendometrial cysts (8%). Surgery was avoided when seven sonohysterograms (14%) revealed normal endometria or subendometrial cysts. In the group with histopathologic correlation, 23 of 28 polyps were confirmed and two of five thickened endometria were shown to represent endometrial hyperplasia. Twelve (63%) of 19 sonohysterogams with prior normal endometrial biopsy findings had abnormalities on sonohysterography, including 10 polyps and two thickened endometria. CONCLUSION. Sonohysterography aids the diagnosis of endometrial abnormalities in tamoxifen-treated women even if prior endometrial biopsies have negative findings. In 14% of cases, visualization of a normal endometrium on sonohysterography obviated surgery.