To ascertain whether uterine malignant mixed müllerian tumors are biologically distinct from high-grade endometrial carcinomas (FIGO grade 3), we compared patient survival in 32 and 39 cases, respectively. The Cox proportional hazard model was employed to determine whether tumor type was an independent predictor of survival. The survival of patients with MMMT was also compared to that of patients with serous adenocarcinoma and clear cell carcinoma. The 5-year overall and disease-free survival were significantly lower for malignant mixed müllerian tumors (25% and 11%) than for high-grade endometrial carcinomas (64% and 56%). Using the Cox proportional hazard model, tumor type (MMMT vs. high-grade endometrial carcinoma) was a statistically significant predictor of survival after other important prognostic variables such as pathologic stage, depth of myometrial invasion, and vascular invasion had been taken into account. The increased aggressiveness of MMMT appears most attributable to their tendency to reach a more advanced stage by the time of clinical presentation and to their greater propensity for upper abdominal dissemination. The survival of patients with MMMT was also lower than that of patients with the special histologic variants of endometrial carcinoma, serous adenocarcinoma and clear cell carcinoma, which are recognized for their unusually aggressive clinical behavior. These results indicate that uterine malignant mixed müllerian tumors are clinically more aggressive than high-grade endometrial carcinomas and should continue to be recognized as a distinct entity.