Purpose: To assess interobserver variation and efficiency of various measurements on 3-dimensional transvaginal ultrasound (3DTVS) and saline infusion sonography (3D-SIS) for preoperative measurement of deep myometrial invasion (DMI) in endometrial cancer. Materials and Methods: One hundred ten women with atypical endometrial hyperplasia or endometrial adenocarcinoma had preoperative 3D-TVS and 3D-SIS. Endometrial thickness (ET), ET in percent of uterine diameter (PAP) and various volumes were measured. AUC, sensitivity, and specificity of most optimal cut-points, and kappa statistics for observer variation were calculated. Results: PAP identified two-thirds of women with DMI. ET and 3D volume measurements had substantial observer agreement compared to moderate agreement of subjective evaluation of DMI. AUC for measurements ranged 0.63–0.73 for identification of DMI. Subjective evaluation of DMI had highest AUC only marginally improved by adding PAP. Conclusion: Preoperative staging of endometrial cancer is not improved by adding 3D volume measurements to subjective evaluation, but PAP could be used to select women with a high risk of DMI for further MRI.