Cytochrome P450 1A2 (CYP1A2) is hypothesized to catalyze the activation of arylamines, known human bladder carcinogens present in cigarette smoke. The relationship between CYP1A2 phenotype and bladder cancer risk was examined in a case-control study involving 519 patients and 514 controls in Shanghai, China. Both CYP1A2 andN-acetyltransferase 2 (NAT2) phenotypic status were determined by a caffeine-based urinary assay. Our study showed that among smokers at urine collection, patients with bladder cancer had statistically significantly higher CYP1A2 phenotype scores compared to control subjects (p= 0.001). The odds ratios (95% confidence intervals) of bladder cancer for the second, third and fourth quartiles of the CYP1A2 score were 1.31 (0.53–3.28), 2.04 (0.90–4.60) and 2.82 (1.32–6.05), respectively, relative to the lowest quartile (pfor trend = 0.003). NAT2 slow acetylation phenotype was associated with a statistically significant 40% increased risk of bladder cancer, and the relationship was independent of subjects' smoking status. Subjects possessing the NAT2 slow acetylation phenotype and the highest tertile of CYP1A2 scores showed the highest risk for bladder cancer. Their odds ratios (95% confidence intervals) was 2.13 (1.24–3.68) relative to their counterparts possessing the NAT2 rapid acetylation phenotype and the lowest tertile of CYP1A2 scores. The findings of our study demonstrate that CYP1A2 phenotype may be an important contributing factor in the development of smoking-related bladder cancer in humans.