Abstract Objective Ovarian carcinosarcomas (OCS) are rare tumors composed of both malignant epithelial and mesenchymal elements. We compared the natural history and outcomes of OCS to serous carcinoma of the ovary. Methods Patients with OCS and serous carcinomas registered in the Surveillance, Epidemiology, and End Results (SEER) database between 1988 and 2007 were analyzed. Demographic and clinical characteristics were compared using chi square tests while survival was analyzed using Cox proportional hazards models and the Kaplan–Meier method. Results A total of 27,737 women, including 1763 (6.4%) with OCS and 25,974 (93.6%) with serous carcinomas, were identified. Patients with carcinosarcomas tended to be older and have unstaged tumors (P<0.0001). After adjusting for other prognostic factors, women with carcinosarcomas were 72% more likely to die from their tumors (HR=1.72; 95% CI, 1.52–1.96). Five-year survival for stage I carcinosarcomas was 65.2% (95% CI, 58.0–71.4%) vs. 80.6% (95% CI, 78.9–82.2%) for serous tumors. Similarly, five-year survival for stage IIIC patients was 18.2% (95% CI, 14.5–22.4%) for carcinosarcomas compared to 33.3% (95% 32.1–34.5%) for serous carcinomas. Conclusions Ovarian carcinosarcomas are aggressive tumors with a natural history that is distinct from serous cancers. The survival for both early and late stage carcinosarcoma is inferior to serous tumors.