We investigated c-erbB-2 oncogene amplification and over-expression in 79 invasive breast carcinoma samples using fluorescence in situ hybridization (FISH) and immunohistochemistry, with the aim of studying relationships between neoplasms over-expressing c-erbB-2 with or without amplification and bio-pathological parameters used in clinical breast cancer. Nineteen samples showed amplification, and all of these were positive by immuno-histochemistry. Moderate or intense immunostaining was present in a further 22 samples without c-erbB-2 amplification and was not related to any increased number of c-erbB-2 signals: 15 samples exhibited chromosome 17 polysomy, 3 monosomy and 4 no FISH abnormalities. Thirty-eight samples were immunonegative: 18 exhibited chromosome 17 polysomy, 9 monosomy and 11 no alterations. Samples having c-erbB-2 over-expression associated with amplification showed DNA aneuploidy and hormonal receptor loss to a greater extent than those expressing c-erbB-2 without amplification or immunonegative samples (chi2 test, p = 0.007, 0.008 and 0.008, respectively). The proliferation rate, detected by Ag-NOR staining, was highest in amplified samples (Kruskal Wallis test, p = 0.009). Our results indicate that tumours showing both c-erbB-2 over-expression and amplification exhibit more aggressive biological characteristics than those with only over-expression or immunonegative tumours. Since both c-erbB-2 amplification and over-expression have been related to negative responses to chemotherapy and poor prognosis, these differences might have clinical implications. The combination of FISH and immuno-histochemistry may be helpful to achieve this aim.