Aim: To predict the myometrial invasion with three-dimensional (3D) ultrasonography in a cohort of patients with endometrial carcinoma by a previously described technique. Material and methods: The moyometrial infiltration was evaluated by 3D ultrasonography before surgery in 54 patients with endometrial carcinoma. After scanning the whole uterus by ultrasonography, three perpendicular planes were identified to find the shortest myometrial tumor-free distance to serosa (TDS) by examining the lateral, anterior, posterior, and fundal parts of the myometrium. Myometrial infiltration was also estimated by the subjective impression of the examiner. The reference standards consist of myometrial infiltration and TDS which are measured by an experienced pathologist. Results: Forty-five patients (age range 45-86 years) were included for the final analysis. Myometrial invasion was <50% in 36 and >= 50% in 9 cases at histologic sections. The TDS which is measured with 3D ultrasonography was positively correlated with histologically measured TDS (r=0.474, p=0.001). The best cut-off value for ultrasonographically measured TDS was 9 mm with a sensitivity of 89%, specifity of 61%, positive predictive value of 36%, and negative predictive value of 96%. Subjective impression has a sensitivity of 100%, specifity of 88%, positive predictive value of 69%, and negative predictive value of 100%. Cervical involvement was correctly identified in all 6 cases by subjective impression. Conclusion: This validation study confirms the 3D ultrasonography as a valuable tool for the evaluation of myometrial infiltration in patients with endometrial carcinoma.