A retrospective study of 103 patients with endometrial sarcomas treated at the University of Michigan Hospital includes 47 mixed homologous tumors, 32 mixed heterologous tumors, 21 pure homologous sarcomas (endometrial stromal sarcoma), and three pure heterologous sarcomas (rhabdomyosarcomas). Clinical characteristics appeared similar among the four patient groups. Fourteen percent of the patients had received previous pelvic irradiation. Twenty-nine percent of the patients with a clinical stage I or II tumor had extrauterine disease discovered at surgery. Extent of the tumor at the time of surgery strongly correlated with outcome, and only two patients with extrauterine disease were long-term survivors. Life table survival probability at five years was 58% with surgical stage I, 33% with surgical stage II, 13% with surgical stage III, 0 with surgical stage IV, and 5% in patients referred with recurrence. Risk factors predicting treatment failure were analyzed separately in surgical stages I and II using Cox model analysis. There was no difference in risk for treatment failure between pure endometrial stromal sarcoma, mixed homologous sarcoma, or mixed heterologous sarcoma. The strongest factor correlating with a poor outcome was deep myometrial invasion (P less than .001). An adverse trend was detected in those patients with previous pelvic irradiation, with advancing patient age, and with increasing uterine size, although these differences did not reach statistical significance. Neither the presence of heterologous tumor elements nor cervical involvement were found to be adverse prognostic factors.