The HER-2/neu oncogene encodes for a specific cell-surface glycoprotein similar to the human growth factor receptor. An analysis of 247 patients with endometrial cancer treated between 1979 and 1983 was performed using an immunoperoxidase technique on paraffin-embedded tissue samples to detect HER-2/neu overexpression. Specimens were graded blindly with regard to HER-2/neu staining intensity. Overexpression of HER-2/neu was identified as strong in 37 patients (15%), mild in 144 (58%), and none in 66 (27%). The 5-year progression-free survival was 56% for the strong, 83% for the mild, and 95% for the nonstaining groups. The strong (P < 0.0001) and the mild (P = 0.028) staining groups were distinct from the nonstaining group in predicting progression-free survival. Likewise, strong overexpression was associated with a poor (51%) overall survival (P < 0.0001). Multivariate analysis revealed that intense overexpression had independent significance in predicting progression-free (P = 0.0003) and overall survival (P < 0.0001). In stage I patients (203), the 5-year progression-free survival was 62% for the strong and 97% for the nonstaining groups (P = 0.0007). This retained independent significance when subjected to multivariate analysis (P = 0.0017). Other significant stage I prognostic factors in multivariate analysis included DNA ploidy, histologic subtype, and histologic grade but not depth of invasion.