PURPOSE: Approximately 25-30% of breast and ovarian carcinomas have amplification of the HER-2/neu oncogene. The aim of the present study was to focus on HER-2/neu gene amplification in different clinical stages of breast cancer in order to (1) determine if fluorescent in situ hybridization (FISH) can be used to detect HER-2/neu gene amplification in different clinical stages of breast cancer, (2) establish whether HER-2/neu gene amplification characterizes a subset of breast cancer in each of these stages, and (3) determine whether a trend for correlation of amplification with the clinical stage of the disease can be detected using the FISH technology. METHODS: A total of 40 specimens of formalin-fixed, paraffin-embedded breast cancer tissues were analyzed cytogenetically, in a blinded fashion, for HER-2/neu gene amplification using FISH and the Vysis LSI HER-2/neu Orange and CEP 17 Green DNA dual color probe. The criterion for "high amplification" was an amplification ratio of >4.0, that for "moderate amplification" a ratio between 2.1 and 4.0, and that for "low amplification" a ratio of 1.5-2.0. RESULTS: Using a cutoff point of > or =1.5, the overall frequency of HER-2/neu gene amplification among stage I tumors was 30% (3 out of 10). Of these, one-third (1 out of 3) showed low amplification, one-third (1 out of 3) were moderately amplified, and one-third (1 out of 3) were highly amplified. The overall frequency of HER-2/neu gene amplification among stage II tumors was 0% (0 out of 10). The overall frequency of HER-2/neu gene amplification among stage III tumors was 10% (1 out of 10). The sole tumor found positive was classified as moderately amplified by our criteria. The overall frequency of HER-2/neu gene amplification among stage IV tumors was 50% (5 out of 10). Four of the 5 tumors found positive were highly amplified. The overall frequency of gene amplification in the 40 cases studied was 22.5% (9 out of 40 tumors studied). CONCLUSION: Although a linear correlation between HER-2/neu amplification and clinical stage cannot be established at this time, it is interesting to note that when stages I and II, and when stages III and IV are combined, respectively, the latter category has a higher amplification frequency than the former. Furthermore, stage IV has the highest frequency (5 out of 10) of HER-2/neu gene amplification than all three lower stages combined (4 out of 30). This is no doubt due to the high frequency of gene amplification observed in stage IV tumors, which, interestingly, also demonstrate high level amplification of HER-2/neu gene copy numbers. Although the biologic and clinical basis for gene amplification is not clear, given the observation that the most aggressive disease stage is associated with the highest frequency of gene amplification and the most high level amplification, further exploration of HER-2/neu as a prognostic marker of poor outcome using FISH is warranted.