Background: A clinicopathologic evaluation of clinical Stage I and II uterine sarcoma was done by the Gynecologic Oncology Group from 1979-1988., Methods: After all eligibility criteria were met, 453 cases were evaluable and analyzed for prognostic factors., Results: Of the 301 mixed mesodermal tumors (MMT), 167 were homologous (HO), and 134 were heterologous (HE). Fifty-nine tumors were leiomyosarcomas (LM). The remaining 93 sarcomas were predominantly stromal cell and adenosarcomas. For this study, only the MMT or LM tumors were analyzed. The recurrence rate for all MMT was 53% (HO, 44%; HE, 63%). The recurrence rate for LM was 71%. The site of the first recurrence included the pelvis in 21% of MMT and 14% in LM. Factors significantly related to progression-free interval (PFI) by univariate analysis among MMT were adnexal spread, lymph node metastases, tumor size, lymphatic-vascular space involvement, histologic grade, cell type, age, peritoneal cytologic findings, and depth of uterine tumor site of invasion. The prognostic factors based on multivariate analysis were adnexal spread, lymph node metastases, histologic cell type (HO versus HE), and grade of sarcoma. For LM, the mitotic index was the only factor significantly related to PFI.