Carcinomas of the endometrium and ovary with undifferentiated components are uncommon neoplasms that are likely underdiagnosed. They are important to recognize as they have been shown to be clinically aggressive. We identified 32 carcinomas with undifferentiated components as defined by Silva and co-workers, 26 endometrial and 6 of ovarian origin. The patient age ranged from 21 to 76 years (median 55); 40% of patients were <or=50 years of age. Most patients (58% of endometrial and 83% of ovarian carcinomas with undifferentiated components) presented at advanced stages (FIGO III-IV). Pelvic and para-aortic lymph nodes were the most frequent sites of metastases. Twenty tumors, entirely undifferentiated, consisted of sheets of dyshesive, ovoid cells with uniform, large vesicular nuclei, whereas 12 tumors contained combinations of differentiated endometrioid adenocarcinoma with undifferentiated components. Although most undifferentiated tumors had a monotonous cytologic appearance without prominent stroma, six showed focal nuclear pleomorphism and eight cases had variably sized zones of rhabdoid cells in a background of myxoid stroma. The tumors were frequently misdiagnosed; they received a wide range of diagnoses, including FIGO grade 2 or 3 endometrioid carcinoma, carcinosarcoma, high-grade sarcoma including endometrial stromal sarcoma, neuroendocrine carcinoma, lymphoma, granulosa cell tumor and epithelioid sarcoma. Up to 86% of the cases showed focal, but strong keratin and/or epithelial membrane antigen staining, with CK18 being the most frequently positive keratin stain. They were predominantly negative for neuroendocrine markers, smooth muscle markers and estrogen receptor/progesterone receptor. Mismatch repair protein expression by immunohistochemistry was evaluated in 17 cases, and 8 (47%) were abnormal (7 with loss of MLH1/PMS2 and 1 with MSH6 loss). Follow-up was available for 27 patients, although it was very short in many cases, ranging from 0.5 to 89 months (median 9 months). Eleven patients (41%) died of the disease in 0.5-20 months, four are alive with disease and twelve patients have no evidence of disease. Endometrial and ovarian carcinomas with undifferentiated components have a broad histologic differential diagnosis, but they show specific histologic features that should enable accurate diagnosis. These tumors can occur in young women, may be associated with microsatellite instability and behave in a clinically aggressive manner.