Background:Diabetes mellitus has been associated with an increased risk of bladder cancer, although the evidence is still open to discussion.Methods:We examined this association using data from a multicentre Italian case–control study, conducted between 2003 and 2014 on 690 bladder cancer cases and 665 frequency-matched hospital controls. Odds ratios (ORs) for diabetes were estimated by unconditional multiple logistic regression models, after allowance for major known risk factors for bladder cancer.Results:One hundred and twelve (16.2%) cases and 57 (8.6%) controls reported a diagnosis of diabetes mellitus, corresponding to a multivariate OR of 2.09 (95% confidence interval (CI): 1.46–3.01). Bladder cancer risk increased with duration of diabetes (OR 1.92 for 1–< 5 years, 1.63 for 5–< 10 years, 2.39 for 10–< 15 years, and 2.58 for⩾ 15 years). The increased risk of bladder cancer was consistent in …