The p15 gene which encodes a cyclin-dependent kinase inhibitor, is located in the 9p21 chromosomal region that is frequently deleted in human bladder transitional cell carcinomas (TCCs). The aim of the present paper is to study the potential involvement of the p15 gene in the evolution of TCCs. p15 mRNA expression was investigated by semi-quantitative RT-PCR in a series of 75 TCCs, 13 bladder cell lines and 6 normal bladder urothelia by semi-quantitative RT-PCR. p15 was expressed in the normal urothelium but p15 mRNA levels were significantly decreased in 66% of the superficial (Ta-T1) TCCs (P = 0.0015). In contrast, in muscle-invasive (T2-T4) TCCs, p15 expression differed widely between samples. p16 mRNA levels were also studied and there was no correlation between p15 and p16 mRNA levels, thus indicating that the two genes were regulated independently. Lower p15 expression in superficial tumours did not reflect a switch from quiescence to proliferative activity as normal proliferative urothelial controls did not present decreased p15 mRNA levels relative to quiescent normal urothelia. We further investigated the mechanisms underlying p15 down regulation. Homozygous deletions of the p15 gene, also involving the contiguous p16 gene, were observed in 42% of the TCCs with decreased p15 expression. No hypermethylation at multiple methylation-sensitive restriction sites in the 5;-CpG island of p15 was encountered in the remaining tumours. Our data suggest that decreased expression of p15 may be an important step in early neoplastic transformation of the urothelium and that a mechanism other than homozygous deletions or hypermethylation, may be involved in p15 down regulation.