An epidemiological association between diabetes mellitus and transitional cell carcinoma of the bladder has been proposed. This study looked retrospectively at 125 patients with transitional cell carcinoma of the bladder as a study group and 80 other hospital patients with conditions not specifically associated with diabetes mellitus as a control group. Diabetic patients had an increased, significant odds ratio for bladder cancer compared with non diabetics even after adjustment for smoking and age [OR: 2.69 p=0.049 (95% CI 1.006–7.194)] A history of smoking OR 2.16 p=0.013 (95% C.I. 1.175–3.964) is a significant independent association with transitional cell carcinoma of the bladder as is age: p=0.001 OR 1.07. We propose potential pathogenic pathways for transitional cell carcinoma of the bladder in diabetic patients based on altered integrin and cadherin distribution in urothelial cells in diabetic patients. A larger study is planned to confirm an association between diabetes mellitus and transitional cell carcinoma of the bladder.