Six endometrioid carcinomas of the ovary and three of the endometrium composed predominantly or entirely of large eosinophilic (oxyphilic) cells are reported. The ovarian tumors occurred in women 31-75 years of age, with a mean of 58 years, and the endometrial tumors occurred in women 37-50 years, with a mean of 44 years. All the ovarian tumors and one endometrial tumor contained focal areas typical of endometrioid carcinoma, with round to oval tubular glands lined by simple or stratified columnar cells and focal squamous differentiation. Two endometrial tumors were composed almost exclusively of oxyphilic cells lining glands. One endometrial tumor contained prominent luminal and intracytoplasmic mucin. Five of the ovarian tumors were grade 2/3 and one was grade 3/3, whereas two of the endometrial tumors were grade 1/3 and one was grade 2/3. The prominence of the oxyphilic cells posed diagnostic difficulty in most of the cases. Electron microscopic examination performed on all tumors showed abundant mitochondria in only one, an ovarian tumor. Other organelles, especially microfilaments and tonofibrils, are cited as other possible reasons for the eosinophilia. Four of the nine cases were recent; follow-up of the remaining five showed a biological behavior similar to the typical endometrioid carcinoma of the ovary and endometrium. We suggest that the diagnosis of "endometrioid carcinoma, oxyphilic cell type" is appropriate for this variant of carcinoma largely composed of eosinophilic cells that may or may not be "oncocytic" in nature. The importance of recognizing this entity lies in distinguishing it from diverse other primary and metastatic oxyphilic cell tumors of the ovary and eosinophilic cell metaplasia and rare other types of primary carcinoma with eosinophilic cells of the endometrium, which may be especially challenging in a curettage or biopsy specimen.