Flow cytometry of various gynecological tumors has shown that aneuploid tumors and a high S-phase rate carry a prognosis worse than that of diploid tumors or tumors with a low S-phase. The aim of this study is to investigate the prognostic importance of DNA ploidy and S-phase rate in relation to mitotic count, tumor stage, tumor grade, and histology in 37 patients with uterine sarcoma stages I-IV (FIGO). Flow cytometry was performed on archival paraffin-embedded tumor tissue and the histologic specimens were reviewed by a single pathologist. Nineteen (51%) of the tumors were classified as DNA aneuploid. The S-phase fraction (SPF) was determined in 33 cases. The mean SPF (+/- SD) was 15.0% (+/- 9.5%). The mean SPF was three times higher in aneuploid tumors than in diploid cases. Both the proportion of aneuploid tumors and the mean SPF were significantly higher in later stage tumors, more poorly differentiated tumors, and tumors with a higher mitotic index. No significant differences were seen between histologic types with respect to the two cytometric variables. The 5-year cancer survival rate was only 11% in aneuploid cases compared with 59% in diploid cases (log rank, P = 0.0002). There was a significantly worse prognosis in cases with a higher SPF (P = 0.0009) and in case with a higher mitotic index (P = 0.0016). In the multivariate survival analysis using the Cox proportional hazards model, DNA ploidy showed a significant prognostic value (P = 0.046) even when adjusted for stage, grade, and mitotic index. When adjusted for stage and grade only, SPF showed significant additional prognostic value.