erbB-2 amplification and overexpression have been suggested as potentially useful prognostic markers in bladder cancer. We examined 141 bladder tumor specimens (45 fresh tissue samples and 96 formalin fixed tissue blocks) for erbB-2 amplification using fluorescence in situ hybridization. A dual labeling hybridization using a repetitive pericentromeric probe specific for chromosome 17 and a cosmid probe for the erbB-2 locus was performed to analyze the erbB-2 copy number in relation to chromosome 17 copy number on a cell by cell basis. Amplification (more than twice as many erbB-2 signals as centromere 17 signals per tumor) was found in 10 of 141 tumors. There was considerable heterogeneity in erbB-2 amplification. In a given tumor there was a wide range of erbB-2 copy number in amplified cells. The arrangement of erbB-2 signals in clusters in all amplified cases suggests that erbB-2 amplification occurs intrachromosomally in bladder cancer. Amplification was found only in tumors with aneusomy of chromosome 17 and was more frequent in pT2-T4 tumors than in pTa/T1 tumors. Overexpression was present without amplification in 51 tumors. All tumors with erbB-2 amplification showed erbB-2 overexpression. However, in 5 samples the proportion of cells with amplification was significantly lower than the fraction of cells with overexpression, indicating coexistence of two different mechanisms leading to overexpression in these tumors.