Inactivation of the retinoblastoma (RB) gene is known to be implicated in the pathogenesis of several types of human cancers. Since structural alterations of the RB gene have not been well examined in human bladder cancer, we looked for mutations in the entire coding region of this gene using polymerase chain reaction (PCR) and single-strand conformational polymorphism analysis of RNA. We also examined allelic loss of the RB gene using PCR-based restriction fragment length polymorphism analysis. Of 30 samples obtained from patients with bladder cancer, eight (27%) were found to have RB gene mutations. DNA sequencing of the PCR products revealed five cases with single point mutations and three cases with small deletions. These mutations included one (10%) of ten low-grade (grade 1) tumours, four (50%) of eight intermediate-grade (grade 2) tumours and three (25%) of 12 high-grade (grade 3) tumours. Likewise, mutations were found in four (21%) of 19 superficial (pTa and pT1) tumours and four (36%) of 11 invasive (pT2 or greater) tumours. In 15 informative cases, loss of heterozygosity at the RB locus was shown in five cases (33%), three cases with RB mutations and two without them. These results suggest that RB gene mutations are involved in low-grade and superficial bladder cancers as well as in high-grade and invasive cancers.