Purpose: To evaluate HER-2 expression and amplification in a large cohort of endometrial cancer with complete surgical staging and outcome data. Patients and Methods: A tissue microarray was constructed of 483 patients with endometrial cancer of diverse histologic type and stage and tested for HER-2 expression and amplification using current standards of practice. There was outcome data for 83% of all patients and 81% with complete surgical staging. Results: Both expression and amplification of HER-2 was associated with high-grade (P = .0001) and high stage (P = .0001) endometrial cancer. The highest rate of HER-2 expression and amplification was seen in serous carcinoma (43% and 29%), while grade 1 endometrioid adenocarcinoma showed the lowest levels (3% and 1%). For all histologic types, the rate of HER-2 expression and amplification was remarkably different (P < .0001) for grade 3 cancers (31% and 15%) versus grade 2 (7% and 3%) and grade 1 cancers (3% and 1%), with similar results for endometrioid type (P < .0001). Both HER-2 expression and amplification correlated with disease-specific survival and progression-free survival in univariate analyses. By multivariate analysis HER-2 expression in the presence of amplification (P = .012) correlated with overall survival, but not expression in the absence of amplification. Overall survival was significantly shorter (P = .0001) in patients who overexpressed (median, 5.2 years) and/or showed amplification of HER-2 (median, 3.5 years) versus those that did not (median of all cases, 13 years). Conclusion: Our results would suggest that HER-2 is an important oncogene in high grade and stage endometrial cancer, but plays only a minor role in the much more common low grade and stage tumors that encompass the majority of clinical practice.