Oncogene amplification and chromosomal anomalies are found in many solid tumors and are often associated with aggressiveness of cancer. We evaluated the frequency and the role of c-erbB-2 gene amplification, relative increase in c-erbB-2 gene copy number, and gain of chromosome 17 in bladder cancer. A total of 29 bladder cancer specimens were examined using fluorescence in situ hybridization (FISH). Dual labeling hybridization with a directly labeled centromere probe for chromosome 17 together with a probe for the c-erbB-2 locus was performed. c-erbB-2 gene amplification was found in 3.4% (1 of 29) of specimens. Relative increase in c-erbB-2 gene copy number was found in 41.4% (12 of 29) of specimens and was significantly associated with tumor grade (P = 0.044 by Fisher's exact test). Gain of chromosome 17 was identified in 65.5% (19 of 29) of specimens and was significantly associated with tumor grade (P = 0.002 by Fisher's exact test) and tumor stage (P = 0.003 by Fisher's exact test). Our results suggest that c-erbB-2 gene amplification, relative increase in c-erbB-2 gene copy number, and gain of chromosome 17 may play important roles in the development and progression of bladder cancers. Moreover, the use of c-erbB-2 amplification, relative increase in c-erbB-2 gene copy number, and gain of chromosome 17 using FISH, together with tumor grade and stage, may provide a more useful clinical indicator in bladder cancer.