Chromosomal instability is common in bladder cancer and could be caused by mutations of mitotic checkpoint genes. Therefore we screened for mutations of the mitotic checkpoint genes hBUB1, hBUB1B, hBUB3 and TTK in six aneuploid bladder cancer cell lines and 15 human bladder tumours. The screening was performed by sequence analysis of the entire coding regions of the four genes. No mutations were detected in any of the four genes. We detected several sequence variations in hBUB1, hBUB1B and TTK both new and previously published. The genetic stability of the four gene loci were tested by loss of heterozygosity (LOH) analysis in the 15 patient samples, showing one LOH for each of the hBUB1B, hBUB3 and TTK loci (6.7%) of the cases, all in different tumour samples. No LOH was detected at hBUB1. We conclude that both mutational inactivation, and loss of one allele, of the examined mitotic checkpoint genes are relatively uncommon.