BACKGROUND AND OBJECTIVES: The standard treatment for endometrial cancer is simple hysterectomy with bilateral salpingo-oophorectomy. Patients with high risk also benefit from lumbo-aortic lymphadenectomy. High risk patients include those with grades and histologic subtypes associated with poor prognosis and depth of myometrial invasion greater than 50% (M2). To determine which patients would benefit from lumbo-aortic lymphadenectomy, the depth of myometrial invasion can be assessed by intraoperative frozen section or by magnetic resonance imaging (MRI). We aimed to determine the diagnostic yield of intraoperative frozen section and MRI for detecting the presence of M2 in patients with endometrial cancer. MATERIALS AND METHODS: This cross-sectional study included women with a histologically confirmed diagnosis of endometrial cancer who underwent baseline MRI and surgical intervention in our institution between 1 January 2010 and 31 December 2017. We reviewed the MRI studies and the intraoperative frozen section reports and compared them with the information in the histopathology report. We compared the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the two tests. We also calculated the diagnostic accuracy of each method and the percentages of underestimation and overestimation. Finally, we calculated the predictive value of MRI for the presence of M2, adjusting it for the histologic variables known to be associated with poor prognosis. RESULTS: To detect M2, MRI had 63% sensitivity, 87% specificity, 73% PPV, and 81% NPV; the diagnostic accuracy was 78.8%, with 13.12% underestimation and 8.13% overestimation of M2. Intraoperative frozen section had 69% sensitivity, 86.7% specificity, 69% PPV, and 86% NPV; the diagnostic accuracy was 81.5%, with 9.24% underestimation and 9.24% overestimation of M2. The degree of concordance between the two methods was moderate (k=0.54, p < 0.00001). CONCLUSIONS: In our experience, MRI and intraoperative frozen section have adequate diagnostic yields for determining M2, though intraoperative frozen section is slightly better. The contribution of MRI in determining the presence and the site of deep myometrial invasion, as well as the factors that can confound the diagnosis, when added to the contribution of intraoperative frozen section, means that both methods help reduce the number of unnecessary lymph node dissections and the morbidity, mortality, and health costs associated with this practice.