Fluorescence in situ hybridization (FISH) to interphase nuclei has been a valuable method for examining the chromosome copies in tumor cells in clinical practice. Twelve cases of transitional cell carcinoma (TCC) of the bladder were investigated with a biotin-labeled repetitive DNA probe to detect numerical aberrations of chromosome 10 in interphase nuclei. The cells containing one fluorescent signal were screened in two of seven non-invasive tumors and in four of five invasive tumors. Two patients presented two FISH spots of different sizes. More than two signals were seen in one invasive tumor. The findings suggest that partial or complete loss of a chromosome 10 is a nonrandom aberration in bladder cancer.