PURPOSE: The main clinical significance of HER-2/neu gene amplification is derived from its use as (a) a prognostic indicator, (b) a predictive factor for the sensitivity to chemotherapy, and (c) the identification of cases that are eligible for a specific therapy targeting the HER-2/neu protein. Over- expression of HER-2/neu has been shown to be associated with a poor prognosis for patients with node-positive breast cancer and also possibly for patients with node-negative breast cancer. The purpose of this work was to analyze the HER-2/neu gene amplification and correlate it with other clinico-pathologic parameters. METHODS: The study population consisted of 194 patients with breast cancer who had been treated with curative surgery at the Kangdong Sacred Hospital, Seoul, Korea from 1995 to 2000. Paraffin-embedded tissue samples from the primary tumors were obtained from the hospital archives and the tissue microarray was then constructed. We analyzed the amplification of HER-2/neu gene by the two-color FISH (Fluorescence in situ hybridization) method, and we correlated the results with other clinico-pathologic parameters such as tumor size, stage, histologic grade, lymph node status and hormonal receptor status. RESULTS: For 44 cases (22.7%) of a total 194 cases, the HER-2/neu gene was amplified. HER-2/neu gene amplification was positively correlated with TNM stage and lymph node status, and it was inversely correlated with estrogen receptor positivity. CONCLUSION: For breast cancer, the analysis of the HER-2/neu gene by FISH based on a tissue microarray may be useful.