Background: Many reported associations between common genetic polymorphisms and complex diseases have not been confirmed in subsequent studies. An exception could be the association between NAT2 slow acetylation, GSTM1 null genotype, and bladder-cancer risk. However, current evidence is based on meta-analyses of relatively small studies (range 23–374 cases) with some evidence of publication bias and study heterogeneity. Associations between polymorphisms in other NAT and GST genes and bladder-cancer risk have been inconsistent. Methods: We investigated polymorphisms in NAT2, GSTM1, NAT1, GSTT1, GSTM3, and GSTP1 in 1150 patients with transitional-cell carcinoma of the urinary bladder and 1149 controls in Spain; all the participants were white. We also carried out meta-analyses of NAT2, GSTM1, and bladder cancer that included more than twice as many cases as in previous reports. Findings: In our study, the odds ratios for bladder cancer for individuals with deletion of one or two copies of the GSTM1 gene were 1·2 (95% CI 0·8–1·7) and 1·9 (1·4–2·7) respectively (p for trend <0·0001). Compared with NAT2 rapid or intermediate acetylators, NAT2 slow acetylators had an increased overall risk of bladder cancer (1·4 [1·2–1·7]) that was stronger for cigarette smokers than for never smokers (p for interaction 0·008). No significant associations were found with the other polymorphisms. Meta-analyses showed that the overall association for NAT2 was robust (p<0·0001), and case-only meta-analyses provided support for an interaction between NAT2 and smoking (p for interaction 0·009). The overall association for GSTM1 was also robust (p<0·0001) and was not modified by smoking status (p=0·86). Interpretation: The GSTM1 null genotype increases the overall risk of bladder cancer, and the NAT2 slow-acetylator genotype increases risk particularly among cigarette smokers. These findings provide compelling evidence for the role of common polymorphisms in the aetiology of cancer. Relevance to practice: Although the relative risks are modest, these polymorphisms could account for up to 31% of bladder cancers because of their high prevalence.