HER-2/neu and c-myc amplification or overexpression have been reported to be associated with poor prognosis in breast carcinoma. The prognostic significance, however, remains somewhat controversial, partly because of discrepancies among different methodologies used for detection of the oncogene amplification or overexpression. Fluorescence in situ hybridization (FISH) has recently been shown to be a useful technique for analyzing genetic alterations in interphase nuclei in various tumors. In this study, FISH was used to quantitate HER-2/ neu and c-myc gene amplification in touch preparations of frozen tissue from 100 node-negative breast carcinomas. HER-2/neu amplification was found to be associated with an abnormal DNA index (P < .001) and tumor size (P < .04). Amplification of c-myc was associated with S phase (P < .0003), abnormal DNA index (P < .003), and a negative estrogen receptor status (P < .01). The coamplification of both oncogenes was strongly associated with an abnormal DNA index (P < .0001) and with tumor size (P < .009). The use of FISH for detection of HER-2/neu gene amplification was 92% concordant with immunocytochemistry (ICC) used for detection of overexpression of HER-2/neu protein. Fifteen of the 100 cases were both amplified for HER-2/neu by FISH and positive by ICC analysis. Seven cases without HER-2/neu gene amplification demonstrated HER-2/neu protein overexpression by ICC. One HER-2/neu-amplified case was negative by ICC. Repeat analysis of a subset of cases showed FISH to be a more reproducible method than ICC in the analysis of HER-2/neu in touch preparations of breast carcinoma. FISH is a rapid and reproducible method that allows the accurate measurement of the level of oncogene amplification within interphase nuclei. The use of FISH should provide a more accurate assessment of the prognostic significance of oncogene amplification in breast carcinoma.