Transrectal ultrasonographic examinations before surgery were performed on 27 patients with stage I endometrial cancer to assess myometrial invasion. The findings were compared with the histopathologic data obtained by surgery. Sensitivity and specificity of transrectal ultrasonography in myometrial invasion of endometrium cancer were 82.6 and 100%, respectively. Therefore, transrectal ultrasonography may be a useful diagnostic tool to determine the myometrial invasion, which is the most important prognostic factor in endometrial cancer.