PURPOSE: The objective of our study was to compare the methods used in the literature to analyze HER-2/neu status on archival breast cancer tissue. Therefore, a series of antibodies was evaluated to assess their immunohistochemical (IHC) sensitivity in correlation to gene amplification determined by fluorescence in situ hybridization (FISH). MATERIALS AND METHODS: HER-2/neu overexpression was studied on paraffin sections of 85 invasive breast cancers using a panel of five monoclonal (9G6, 3B5, CB11, TAB250, GSF-HER2) and two polyclonal antibodies (A8010, A0485) in addition to the HercepTest (DAKO, Glostrup, Denmark). HER-2/neu gene amplification was determined by FISH using a dual-color probe (PathVysion; Vysis, Stuttgart-Fasanenhof, Germany). RESULTS: HER-2/neu overexpression was demonstrated in 26% (9G6, TAB250, GSF-HER2), 27% (3B5, CB11), 33% (A8010) and 42% (A0485, HercepTest) of the tumors. FISH on paraffin sections identified gene amplification in 28% of the tumors. Strongly positive IHC results (3+) were always associated with gene amplification. Among the 16 tumors presented with weakly positive IHC results (2+) using the HercepTest, 12 (75%) lacked gene amplification. CONCLUSION: The comparison of IHC and FISH demonstrated an excellent correlation of high-level HER-2/neu overexpression (3+) with gene amplification; ie, FISH does not provide further information in these tumors. However, weakly positive IHC results (2+) obtained with the HercepTest share only a minor association with gene amplification.