Determination of HER-2/neu oncogene amplification has become clinically important in the present management of breast cancer and may have important applications in other areas of clinical oncology and scientific research. In situ hybridization is an extremely accurate and sensitive technique for assessing amplification of HER-2/neu. A new method using a chromogen-labeled probe offers numerous advantages, including the ability to view the morphologic features of the cells of interest using a light microscope, which can be found in every laboratory. We used both techniques to assay 31 cases of infiltrating breast carcinoma; assays were performed in laboratories at 2 institutions. Identical results for both methods were found in 26 cases (10 amplified, 16 nonamplified). One case was misinterpreted as overexpressed by chromogenic in situ hybridization (CISH) because of background precipitate. In 4 cases, CISH suggested low-level amplification. Three of these cases subsequently were found to have chromosome 17 polysomy. In the remaining case, the initial section chosen was suboptimal, showing weak signals by both methods. If a probe for chromosome 17 (now available for CISH) is used in cases of questionable HER-2/neu amplification, CISH seems to be as accurate and more practical than FISH.