In experimental animals such as rats and rabbits, CYP4B1 has an important role in mutagenic activation of procarcinogens in bladders. In human bladders, it is not clear whether CYP4B1 has such role or not. In the present study, human bladder microsomes activated a-aminofluorene which is a typical substrate for CYP4B1 and is a bladder carcinogen, CYP4B1 was detected in the human bladder microsomes by immunoblotting. Furthermore, we developed a microassay for CYP4B1 mRNA by performing real-time RT-PCR. Using this method, CYP4B1 mRNA levels were assayed in transurethal resection samples from the bladders of patients with bladder tumors. The bladder-tumor patients had a significantly higher expression of CYP4B1 than the nonbladder tumor patients, These findings suggest that a high expression of CYP4B1 increases the risk of bladder tumor by activation of carcinogenic aromatic amines, This approach could be an important tool in the assessment of human bladder cancer risk,