OBJECTIVE. The purposes of this study were to compare transvaginal sonography (TVS), intraoperative sonography (IOS), and gross visual inspection of the uterus with the histopathologic findings in patients with endometrioid adenocarcinoma, and to compare the accuracies of TVS, IOS, and gross visual inspection in staging of the tumor. SUBJECTS AND METHODS. Sixteen patients with endometrioid carcinoma were prospectively evaluated with TVS and IOS. Intraoperative gross visual inspection was also performed. Gray-scale, duplex, and color Doppler findings were used to stage patients. The location and depth of myometrial invasion and the presence of cervical involvement were recorded. At gross visual inspection, only the absence or presence end the depth of myometrial invasion (≤58% or >50%) were recorded. The data were analyzed three ways. First, in uterine specimens with myometrial invasion, a site-by-site comparison was made among the TVS and IOS findings and the final histologic results regarding location and depth of tumor invasion. Next, to determine tumor stage, myometrial invasion was defined in two ways: (1) absent, 50% or less, or greater than 50%; and (2) 50% or less or greater than 50%. Then imaging findings, gross visual inspection, and the final histologic results were compared. RESULTS. Of the 16 uterine specimens, eight had myometrial invasion, with 13 separate sites of tumor invasion. IOS correctly identified the location and depth (±10% of the histologic depth) of tumor invasion at four (31%) sites, and TVS at one (8%) site. TVS and IOS overestimated myometrial invasion due to adenomyosis, bulky intraluminal tumor, and lymphovascular invasion. When myometrial invasion was defined as absent, 50% or less, or greater than 50%, TVS was correct in 60% of cases, IOS in 56%, and gross visual inspection in 53%. When myometrial invasion was defined as 50% or less or greater than 50%, TVS was correct in 93% of cases, IOS in 81%, and gross visual inspection in 80%. CONCLUSION. TVS and IOS are inaccurate in predicting the precise location and depth of myometrial tumor invasion. However, when a less rigorous definition of invasion is used, the accuracies of TVS and IOS are comparable to gross visual inspection in staging of the tumor.