The purpose of this study was to correlate the sonographic and histologic findings of focal lesions of the endometrium as depicted by transvaginal sonography. Sixteen focal endometrial masses were encountered during an 8 month period. The transvaginal sonographic appearance was correlated with findings at surgery or hysteroscopy. All uniformly hyperechoic endometrial masses were benign endometrial polyps. Five heterogeneous lesions were complicated endometrial polyps; one of these showed a focus of endometrial carcinoma. Four small and uniformly hypoechoic lesions were all prolapsed submucosal leiomyomata. Our results showed that a well-defined, uniformly hyperechoic mass within the endometrial cavity is most suggestive of a benign endometrial polyp. Larger masses with a heterogeneous appearance are nonspecific and warrant further evaluation and biopsy.