Background:Protooncogene cerbB2 is located in chromosome 17 region q21 and codifies a 185 Kd protein, with tyrosine kinase activity. The amplification of this gene is associated with relapse and lower survival in breast cancer. Overexpression of this gene can be detected by immunohistochemistry (IHC). However, fluorescence in situ hybridization (FISH) and chromogenic in situ hybridization (CISH) allow the simultaneous analysis of morphology and overexpression of the gene. Aim: To evaluate the relationship of c-erbB2 oncogene amplification measured by FISH with histological graduation, presence of positive Iymph nodes and evolution of breast cancer. Patients and methods: One hundred and ten tissue samples of invasive ductal or lobulillar breast cancer, positive for cerbB2 oncogene by IHC were analysed. The presence of c-erbB2 oncogene amplification was subsequently analyzed by FISH. Results: There was a significant association of c-erbB2 amplification by FISH with pathological graduation of the tumor, number of regional Iymph nodes involved and disease free survival. Conclusions: Proto-oncogene cerbB2 amplification is a good indicator of bad prognosis in invasive breast cancer (Rev Méd Chile 2005; 133: 151-7)