Endometrial oncocytic carcinoma is an unusual neoplasm, with few cases reported. Endometrial curettage specimens coded as prominent oxyphilic metaplasia (N = 5) and oxyphilic or oncocytic carcinoma (N = 4) were reviewed, and hysterectomy slides from the four carcinomas were also examined. Immunohistochemical and ultrastructural analyses were performed in three of five metaplasias and in all four carcinomas. Most patients (89%) with oncocytic metaplasia and carcinoma had vaginal bleeding. Oncocytic metaplasia was characterized by a single layer of cells with abundant eosinophilic, granular cytoplasm, minimal pleomorphism, and rare mitotic activity. Carcinoma was diagnosed on the basis of an altered stroma (n = 2) and/or a confluent growth pattern (n = 4) and had a papillary (n = 4), glandular (n = 2), or solid (n = 1) morphology. Carcinomas showed a similar population of oncocytic cells as metaplasias, but with occasional nuclear stratification and greater pleomorphism and mitotic activity. Tumors were International Federation of Gynecology and Obstetrics (FIGO) grade 1 (n = 2) or 2 (n = 2) and FIGO stage Ib, Ic, IIb, and IIIc. Omental metastases developed in the patient with the stage III tumor at 13 months; the two patients with stage I tumors were alive with no evidence of disease at a mean of 29 months. All carcinomas expressed p53 and 75% and 100% were estrogen receptor (ER)- and progesterone receptor (PR)-negative, respectively, whereas all metaplasias were p53 negative- and ER- and PR-positive. Ki-67 labeling index was 1 to 3% in metaplasias and 14 to 33% in carcinomas. Oncocytic metaplasias and carcinomas contained abundant mitochondria and free ribosomes, accounting for the oncocytic appearance. Because oncocytic carcinomas frequently show deep myometrial invasion and require surgical staging, it is important to distinguish oncocytic metaplasia from carcinoma on biopsy material. Ki-67, p53, and ER and PR immunostains may assist in this potentially difficult differential.