We report an unusual case of endometrial adenocarcinoma in a 80-year-old woman who underwent mastectomy for breast cancer at 68 years of age. The tumor diffusely involved the entire thickness of the myometrium. Atypical cells throughout the tumor contained abundant oxiphilic cytoplasm and were arranged in a solid or solid-tubular nests in a focal papillary manner. Components of the carcinoma were focally observed in situ. The tumor was classified according to the International Federation of Gynecology and Obstetrics (FIGO) as grade 2 and stage IIIa. Immunohistochemistry revealed that the tumor cells were positive for p53 but negative for vimentin, estrogen, and progesterone receptors, GCDFP-15, and mammaglobin. They were positive for antimitochondrial antigen and thyroid transcription factor-1. The Ki-67 labeling index was approximately 50%. Immunostaining revealed endometrial oncocytic carcinoma. Distinguishing between primary uterine neoplasm and carcinoma caused by metastasis of breast cancer appears important.