PURPOSE: We evaluated magnetic resonance imaging (MRI) findings of dedifferentiated endometrial carcinoma (DEC), comprising undifferentiated carcinoma and low-grade endometrioid carcinoma. MATERIALS AND METHODS: We recruited 11 patients with pathologically proven DEC treated at our institute. We evaluated primary lesion size, location and signal intensity on MRI, and prognosis. MRI and pathological findings were compared in eight resected patients. RESULTS: Primary tumors ranged from 16 to 206 mm in diameter. DEC was located at the endometrium in 9 of the 11 patients; the remaining two patients showed diffuse involvement of the enlarged myometrium. These two patients with diffuse involvement type died within 4 months. Of the eight patients who underwent resection, seven had macroscopic intratumoral hemorrhage and six showed a high signal on T1-weighted images or low signal on T2-weighted images. Of the eight resected patients, four had tumor necrosis > 25% and tumor size > 5 cm. In these patients, necrosis appeared as nonenhanced areas on contrast-enhanced MRI. CONCLUSION: MRI findings of DEC showed two patterns: mass-forming type and diffuse myometrial type with poor prognosis. Most patients with DEC had intratumoral hemorrhage, and large tumors (> 5 cm) had gross necrosis, which appeared as nonenhanced areas on contrast-enhanced MRI.