Centrosome amplification, which may accelerate tumor progression through chromosomal instability, is frequently observed in human malignancies. The intercellular relation between the number of centrosomes and chromosomes, however, is poorly understood. Therefore, the relationship between centrosomes and chromosomal copy number in the same cells was investigated in bladder cancer. Centrosomes were evaluated by immunohistochemistry, using anti-gamma-tubulin antibody in eight bladder cancer cell lines. Fluorescence in situ hybridization with centromeric probes for chromosomes 7, 9, and 17 was then performed on the same cells stained with gamma-tubulin. The number of centrosomes was directly proportional to the number of chromosomes in cells with centrosome amplification, while a large intercellular variation in chromosomal copy number was detected in cells with normal numbers of centrosomes. Cancer cells with centrosome amplification of even centrosome numbers had significantly more even numbers of chromosomes. In cancer cells with four centrosomes, even numbers of chromosomes were detected more frequently (87.5%). These bladder cancer cell lines showed Aurora-A and p53 overexpression. These data indicate the occurrence of centrosome amplification with the possible mechanism of cytokinesis failure, resulting in a doubling of the number of centrosomes and chromosomes.