Amplification of the HER-2/neu oncogene was determined in 362 tumors from patients with primary breast cancer (185 node-positive patients and 177 node-negative patients). The overall amplification rate was 33% (30% for node-negative patients; 31% for patients with 1-3 positive nodes; 40% for patients with greater than 3 positive nodes). Gene copy number was not associated with axillary lymph node status, steroid receptor status, or patient age but was weakly correlated with the size of the primary tumor. Amplification of the HER-2/neu gene did not correlate with either disease-free or overall survival in univariate or multivariate analyses. The results were unambiguously negative for patients with node-negative disease. Although the univariate results for node-positive patients were marginally significant (P = 0.07), the significance was not retained in multivariate analyses. Thus, while HER-2/neu amplification may be biologically important in primary breast cancer, it will only be of marginal utility as a prognostic factor for predicting clinical outcome.