We examined the histologic, immunohistochemical, and clinical features of a series of 23 endometrial, five cervical, and one ovarian carcinosarcomas (malignant mixed müllerian [mesodermal] tumors) and nine associated distant peritoneal metastases. The primary tumors all showed epithelial differentiation (cytokeratin and/or epithelial membrane antigen expression) of the carcinomatous component, while sarcomatous areas showed epithelial differentiation in all but one case. The metastases showed uniform staining for cytokeratin (eight of eight cases) and epithelial membrane antigen (eight of eight cases), including the spindle cell component that was present in four of nine cases. Desmin significantly changed the interpretation of rhabdomyosarcoma differentiation by refuting putative rhabdomyoblasts in two cases and identifying rhabdomyoblasts in two other cases where they were unrecognized on hematoxylineosin staining. S100 protein was positive in all five cases with chondrosarcoma differentiation. Muscle-specific actin and vimentin were positive in the sarcomatous component of all cases and in the carcinomatous component of seven and 10 cases, respectively. After immunostaining, heterologous elements were present in 18 of 29 cases (11 cases of rhabdomyosarcoma, three cases of chondrosarcoma, three cases of mixed rhabdomyosarcoma and chondrosarcoma, and one case of liposarcoma). Only six of 27 patients with follow-up were disease free for 12 months or longer (associated with stage I or II disease, smaller size, no lymphatic invasion in the resection specimen, and no invasion of the outer two thirds of myometrium). Presence and type of heterologous elements, grade of sarcomatous or carcinomatous components, histologic type of carcinomatous component, gross appearance, presence of necrosis, or use of chemotherapy or radiotherapy did not affect outcome. Carcinosarcomas are clinically aggressive distinctive mixed epithelial-stromal neoplasms with histologic and immunohistochemical features that overlap with metaplastic carcinoma in many cases.