Carcinosarcomas (malignant mixed Mü llerian tumors) of the uterus are rare and aggressive malignancies consisting of an epithelial (carcinoma) and a mesenchymal (sarcoma) tumor component and are considered as metaplastic endometrial carcinomas. This study evaluated molecular characteristics and clinical behavior of uterine carcinosarcomas to improve treatment regimens in the future. Immunohistochemical expression of estrogen receptor-a and -b, progesterone receptor-A and -B, MLH1, MSH2, MSH6, PTEN (phosphatase and tensin homolog deleted on chromosome 10), p53, b-catenin and cyclin D1 was determined in 40 uterine carcinosarcomas. Immunostaining was compared between epithelial and mesenchymal tumor components. To determine the prognostic role of the epithelial component, clinicopathological data and survival were compared between patients with endometrioid and non-endometrioid epithelial tumor components. To determine prognosis of carcinosarcomas compared with high-risk endometrial carcinomas, clinicopathological characteristics and survival were compared between these patients. Hormone receptor expression occurred infrequently: estrogen receptor-a (8%) and -b (32%), progesterone receptor-A (0%) and -B (23%), next to b-catenin (4%) and cyclin D1 (7%). PTEN, MLH1, MSH2 and MSH6 mutations occurred in 39%, 33%, 22% and 21%, respectively (based on absent immunostaining). Overexpression of p53 was observed in 38%. Expression patterns of p53, MSH2 and MSH6 corresponded between epithelial and mesenchymal tumor components. In our cohort, the epithelial component caused the majority of metastases (72%) and vascular invasion (70%). Survival tended to be worse for patients with a non-endometrioid epithelial component compared with an endometrioid epithelial component (5-year survival: 26% and 55%, respectively). Survival was worse for patients with uterine carcinosarcomas compared with high-risk endometrial carcinomas (grade 3 endometrioid and non-endometrioid); 5-year survival rates: 42%, 77% and 57%, respectively. Our results support the monoclonal origin of uterine carcinosarcomas. The epithelial component determines prognosis by causing the majority of metastases and vascular invasion. To improve prognosis, treatment should focus on the epithelial tumor component of uterine carcinosarcomas. Modern Pathology (2011Pathology ( ) 24, 1368Pathology ( -1379 doi:10.1038/modpathol.2011; published online 13 May 2011Keywords: carcinogenesis; clinical behavior; epithelial component; immunohistochemistry; uterine carcinosarcomas Uterine carcinosarcomas (malignant mixed Mü llerian tumors) are rare malignancies of the female genital tract, accounting for 1-5% of uterine malignancies. 1,2 Microscopically, carcinosarcomas consist of two histological malignant components: an epithelial (carcinoma) and a mesenchymal (sarcoma) component. The epithelial component is usually a