The amplification and overexpression of Her2 proto-oncogene have been found to be associated with the development and progression of human breast cancer. A polymorphic valine allele at codon 655 of the Her2 gene (Her2(V655)) was suggested by some authors to be a susceptible genetic factor for the development of breast cancer. The Her2 polymorphism at codon 655 was investigated in 304 Korean women including 177 patients with breast cancer. The association between Her2 genotype and Her2 protein overexpression was also examined in breast cancers by immunohistochemistry. Her2(V655) was not associated with a significant breast cancer risk (odds ratio (OR), 1.792; 95% confidence interval (CI), 0.459-6.991). The frequency of homozygous or heterozygous valine allele increased in stage 2 patients (OR, 1.67; 95% CI, 0.67-4.19), and patients in stages 3 and 4 (OR, 3.36; 95% CI, 0.85-13.42) compared to patients in stage 0. However, an association between the presence of the valine allele and the overexpression of Her2 protein could not be demonstrated. These results suggest that Her2 polymorphism at codon 655 is not associated with the development of breast cancer in Korean women. However, there is a possibility that the valine allele at codon 655 might be related to increased risk of breast cancer progression.