Prognostic factors capable of detecting potential for aggressive disease in early stage endometrial cancer might be useful in selecting patients for early adjuvant therapy. Sixty-three patients with surgical Stage I endometrial carcinoma treated by hysterectomy with a mean follow-up of 55 months were evaluated for tumor type, grade, depth of myometrial invasion, presence of vascular invasion, DNA ploidy, and HER-2/neu overexpression by immunohistochemical techniques. These results were compared with HER-2/neu gene amplifications evaluated by fluorescence in situ hybridization (FISH) and their ability to predict disease survival. For FISH, sections 5 microns thick of formalin-fixed, paraffin-embedded tissues were processed using the Oncor Chromosome In Situ Hybridization System. Automated hybridization using the Ventana Gen was performed with the Oncor unique sequence digoxigenin-labeled HER-2/neu DNA probe. Gene copy numbers were evaluated using the Zeiss Axioskop50 fluorescence microscope. HER-2/neu amplification was noted in 24 (38%) of 63 cases. By multivariate analysis, only aneuploidy (P = .04) and HER-2/neu amplification by FISH (P = .04) independently correlated with survival. Although we saw a relationship between HER-2/neu protein expression and gene amplification, this trend did not achieve statistical significance. HER-2/neu oncogene amplification can be assessed using automated FISH on formalin-fixed, paraffin-embedded tissue. HER-2/ neu amplification predicts poor outcome in Stage I endometrial cancer. HER-2/neu amplification status has potential use in the identification of patients with high risk of disease recurrence who might benefit from intensified therapy.