PURPOSE: Breast cancer is a common malignant tumor for Korean women. Various oncogenes have been demonstrated in malignant tumor cells. There were many reports on the relationship between the prognosis and the tumor markers, and particularly for HER-2/neu; however, this relationship is controversial. The aim of this study was to perform immunohistochemical staining for the HER-2/neu antigenic protein in breast cancer patients to evaluate the expression rate of this antigenic protein, and to compare the correlations with the patients' prognosis. METHODS: The medical records of 100 patients who were diagnosed with breast cancer and who were treated with mastectomy between January 1996 and December 1999 at Soonchunhyang University Cheonan Hospital were selected according to the tissue condition of the paraffin block fixations. The prognostic factors were then investigated. The immunohistochemical expression of HER-2/neu was examined and compared with the survival rate using a Kaplan-Meier estimate and a log rank test. RESULTS: One hundred patients were included in this study, had a mean (SD) age of 47.9 (plus or minus 10.5) years and a follow-up duration of 59.9 (plus or minus 13.3) moths The overall mortality was 35% (35/100) and the mean survival time (SD) was 64.4 (plus or minus 12.0) months. The overall 5-year survival rate was 77.0%. Among the study variables, the tumor stage was a significant predictor of survival, and it as associate with a significantly low survival rate for stage III tumor. Lymph node metastasis, hormonal receptor status, histologic grade and HER-2/neu overexpression were significantly associated with the survival rate for patients with breast cancer. CONCLUSION: HER-2/neu is a very useful prognostic factor, and statistical significance was achieved for this factor.