OBJECTIVE To compare the prognostic importance of certain molecular biologic characteristics (HER-2/neu and p53 gene overexpression, DNA ploidy, and the S-phase fraction) to standard clinical-pathologic factors used to predict survival in patients with endometrial carcinoma. METHODS We reviewed archival specimens from 128 patients with endometrial cancer diagnosed during the period 1985-1987. One hundred four cases were eligible for inclusion in the study. Immunohistochemistry was used to detect p53 and HER-2/neu overexpression. We used flow cytometry to calculate DNA ploidy and the S-phase fraction. Life-table analysis and Cox multiple regression were used to analyze clinical and molecular data with respect to survival. RESULTS International Federation of Obstetrics and Gynecology stage, nuclear grade, lymph-vascular space invasion, and adverse histopathologic features each significantly correlated with poor outcome (each at P < or = .001). Overexpression of p53 was demonstrated in 15% of the tumors and was associated with a 12% probability of 5-year survival, compared to a 90% probability of 5-year survival for the p53-negative cohort (P = .0001). Thirty percent of the tumors were aneuploid, which was also associated with poor prognosis (P = .0003). HER-2/neu overexpression and an S-phase fraction greater than 10% showed similar trends, but were not statistically significant. On multivariate analysis, p53 overexpression was the strongest independent prognosticator of survival (P = .0001). CONCLUSIONS Molecular characteristics provide objective data that may be useful in predicting prognosis in patients with endometrial cancer. Further investigation of molecular and genetic characteristics are needed to refine our diagnostic and treatment modalities.