Monoclonal therapies could represent baseline-personalized medicine for patients with neoplasia. One of the most successful examples is Trastuzumab, a humanized antibody against epidermal growth factor receptor 2. Human epidermal growth factor receptor 2 (HER2) is a trans-membrane tyrosine kinase coded by the gene HER2/neu and overexpressed in approximately 12% to 20% of infiltrating breast carcinomas. The overexpression of HER2 is an independent adverse prognostic factor in relation to survival and is also predictive of response to treatment. Therefore, the correct evaluation of HER2 status is essential for the management of infiltrating breast carcinoma to determine the response to Trastuzumab. The most common evaluation technique is immunohistochemistry, which is confirmed by fluorescent or chromogenic monochrome or dual-gene in situ hybridization in ambiguous cases (immunohistochemical 2+). Our objective was to evaluate the diagnostic value of a new technique on the basis of HER2 mRNA in situ hybridization (HistoSonda) and study its correlation with immunohistochemistry and dual-chromogenic in situ hybridization (DUO-CISH) in 403 cases of infiltrating breast carcinoma. The percentage of DUO-CISH amplification was 25.8%, HistoSonda positivity was 31.2%, and positivity for Hercep-Test was 48.1%, including (+2) and (+3). Comparisons were made of each of the techniques, HistoSonda to IHQ and HistoSonda to DUO-CISH. The overall concordance between DUO-CISH and HistoSonda was 89%. Our data support the consistency of HistoSonda as a useful tool to determine HER2 status in breast cancer.