Myometrial invasion greater than 33% negatively affects the prognosis of endometrial carcinoma. Since the endometrium is readily differentiated from myometrium via high-resolution transvaginal sonography (TVS), this prospective study was undertaken to evaluate the efficacy of TVS in determining the depth of myometrial invasion in women with endometrial adenocarcinoma. Eighteen subjects underwent TVS utilizing 5.0- and 7.5-MHz probes by a single examiner blinded to stage and grade of adenocarcinoma. Predicted TVS ratios were categorized as less than 33% or greater than or equal to 33% and compared to actual histologic invasion. Ultrasound predicted that TVS ratios greater than or equal to 33% are significantly associated with deep (greater than 33%) histologic invasion (P less than 0.01, Fisher's test). When histologic invasion was greater than or equal to 33%, TVS was 100% accurate with no false negatives. The two cases in which TVS ratios erroneously indicated invasion greater than or equal to 33% contained adenomyosis and leiomyomas. TVS is a highly accurate and convenient method for preoperatively evaluating myometrial invasion. Potentially this evaluation could influence the selection of therapy for poor-surgical-risk candidates or direct appropriate referral of patients with deeper invasion to a gynecologic oncologist.