A limited number of previous studies have indicated a low frequency of chromosome 10 allele losses and deletions in bladder cancers. We investigated the involvement of chromosome 10 in advanced bladder cancers. Loss of heterozygosity (LOH) was analysed in 19 microsatellite loci in 20 grade III invasive transitional cell carcinomas. Nine (45%) of the 20 tumors had at least one allele loss on the long arm of chromosome 10. The short arm of chromosome 10 was not affected. The most frequent LOH occurred at D10S215, where four (29%) of 14 of the informative cases had an allele loss. The minimal region with allele losses was located between the centromeric marker D10S1644 and the telomeric marker D10S541, which are separated by 2.52 cM. The results strongly suggest the existence within that region of a tumor suppressor gene or genes for advanced bladder cancer.