Fluorescence in situ hybridization (FISH) with the UroVysion probe set (Abbott Molecular, Des Plaines, IL) was used to assess 31 bladder cancers for chromosomal abnormalities, including 4 adenocarcinomas, 5 urachal adenocarcinomas, 6 small cell carcinomas, 7 squamous cell carcinomas, and 9 typical urothelial carcinomas. FISH was also used to assess the benign urothelium in 4 cases. There was a significant increase (P < .001) in the mean number of chromosome 3 (2.64 vs 1.51), chromosome 7 (2.61 vs 1.48), and chromosome 17 (2.41 vs 1.41) centromeric signals observed in cells from patients with cancer compared with patients without cancer. Of the 31 tumors, 29 (94%) demonstrated polysomic signal patterns in more than 10% of cells. In the 2 remaining tumor specimens, there was a high percentage of cells (>75%) demonstrating homozygous 9p21 deletion. The data from this study suggest that chromosomal abnormalities detectable by FISH in urothelial carcinoma are also common in rarer histologic variants of bladder cancer.