Uterine carcinosarcomas (CSs) are aggressive neoplasms, with 5-year overall survival (OS) rates of less than 35%. They are customarily separated into types harboring either heterologous or homologous mesenchymal elements, but the prognostic significance of this finding is controversial. Our goal was to study clinicopathologic features of possible prognostic relevance in surgical stage I uterine CS. A retrospective clinical and histopathologic review was performed for all women diagnosed with surgical stage I uterine CS. These tumors were compared with stage I high-grade endometrial (HGEm) carcinomas for clinical outcomes. There were 42 cases of surgical stage I uterine CS identified between January 1990 and January 2004. The disease-free survival and OS rates for patients with stage I CS were significantly worse compared with stage I HGEm (P=0.001; P=0.01). The median disease-free survival for patients with heterologous CS was 15 months and had not been reached for women with homologous CS (P=0.001). The 3-year OS rates were 45% versus 93% in women with heterologous compared with homologous stage I CS (P<0.001). The 3-year OS rates for homologous CS and HGEm were both >90%. Homologous stage I CSs have survival outcomes that are similar to HGEm. This further supports the concept that homologous stage I CSs are carcinomas with sarcomatoid features, not sarcomas. More importantly, the presence of heterologous sarcomatous elements is a powerful negative prognostic factor in surgical stage I uterine CS.