Objective: The sensitivity and specificity of immunohistochemical (IHC) methods for HER2 testing are very important given the therapeutic implications. This study compares the concordance between Oracle and the HER2 Dako polyclonal antibody (HER2), in breast carcinomas (BCs) that were equivocal (2+ or not evaluable/NE), by HER2 staining, considering as gold standard the chromogenic in situ hybridization (CISH). Method: BCs (n=34), problematic by HER2 staining (1−2/ 2+ or NE), and studied by CISH in our institution, and 24 additional consecutive BCs were stained with HER2 and Oracle, and scored separately by three pathologists. Consensus scoring for each IHC method and CISH results were recorded. Descriptive statistics and measurement of the Cohen's Kappa coefficient were performed. Results: The overall agreement between the 2 tests in a 3×3 analysis shows a concordance in 62 % of cases (κ=0.405). Among the cases studied by CISH, equivocal were 26 and 11 cases, for HER2 and Oracle, respectively. Seventeen HER2 equivocal cases were negative with Oracle and CISH. In problematic cases, using CISH as gold standard, the sensitivity, specificity, positive and negative predictive values for HER2 and Oracle were 100, 20.7, 17.9, 100 and 100, 72.4, 38.5, 100, respectively. Conclusion: In problematic cases, Oracle testing shows higher specificity and positive predictive value.