We examined 60 cases of human urothelial carcinomas (27 superficial and 33 deeply invasive) for the frequency of p53 gene mutations. Forty-two cases were analyzed by both the immunohistochemical and the single-strand conformation polymorphism (SSCP) methods, and the remaining 18 cases were assayed by SSCP alone. For the latter assay, exons 4 to 8 were amplified by polymerase chain reaction, and the amplified nucleotides were analyzed for the evidence of mutations by gel electrophoresis. When mobility shift was observed, direct nucleotide sequencing was performed to determine mutation sequence. Three superficial and eight deeply invasive carcinomas demonstrated evidence of mutations. Mutations involved various codons randomly. The fact that all tumors with mutations of the p53 gene except for one were of high nuclear grade (grade III) suggests that p53 mutation is associated with the progression of bladder cancers. Our results indicate that SSCP is a sensitive screening assay for detecting gene mutations. Immunohistochemical analysis is also a sensitive method but may yield false positive as well as false negative results.