OBJECTIVE: Pioglitazone, a thiazolidinedione, was approved for treatment of Type 2 diabetes. However, several observational studies suggest an association of pioglitazone with an increased risk of bladder cancer in patients with diabetes. Therefore, we sought to perform a systematic review and meta-analysis to evaluate the magnitude of this association and the quality of the supporting evidence. RESEARCH DESIGN AND METHODS: Electronic databases were queried to identify controlled studies of pioglitazone that measured the risk of bladder cancer. RESULTS: Six studies involving 215 142 patients using pioglitazone were included, with a median period of follow-up of 44 months. The hazard of developing bladder cancer was significantly higher in patients using pioglitazone (hazard ratio 1.23; 95% CI 1.09-1.39; I(2)  = 0%) compared with control groups. The risk of bias was moderate across the six studies. Considering an incidence rate of 20.8 per 100 000 person years, the number needed to harm was five additional cases of bladder cancer per 100 000 person years. CONCLUSION: Pioglitazone use is associated with an increased risk of bladder cancer. Clinicians must consider this increased risk in the context of the baseline risk for bladder cancer of their patients, other adverse features of this medicine (e.g. weight gain, cost) and its potential anti-hyperglycaemic benefits. © 2013 The Authors. Diabetic Medicine © 2013 Diabetes UK.