We previously reported the results of 29 relapsed and 31 non-relapsed cases of node-negative breast cancer studied for HER-2/neu oncogene amplification using FISH and the Vysis HER-2/neu and chromosome 17 control probes (Vysis, Downers Grove, IL). Previous research performed in this laboratory on HER-2/neu amplification in stages I to IV breast cancer indicated that the most aggressive disease stages were associated with the higher frequency of oncogene amplification and the highest level of amplification. These data support the hypothesis that HER-2/neu is a prognostic marker of poor outcome. Thus, a retrospective study of HER-2/neu amplification in relapsed and non-relapsed node-negative breast cancer was initiated to explore further the above question. Out of a total of 46 relapsed cases studied, 13 (28.3%) were found to be amplified. The corresponding frequency of amplification for the controls was 4 (8.7%) of 46 non-relapsed cases. The difference was statistically significant with a p value of <0.05. Although these results still need to be confirmed and extended in other centers, they are quite remarkable and suggest an association between HER-2/neu amplification and disease relapse in patients with node-negative breast cancer. Our results also demonstrate that FISH is a sensitive technique for studying oncogene amplification in archival pathological specimens. (This study was partially funded by Vysis, Inc., Downers Grove, IL).