The frequency with which diabetes mellitus was mentioned on the death certificates of 18,733 patients dying from bladder cancer has been compared with that of 19,709 patients dying from other cancers (excluding cancer of the lung and pancreas). The estimated relative risk of bladder cancer in diabetics was 0-98 with 95% confidence limits 0-70-1-38. There was no increase in risk of bladder cancer in patients with diabetes of long duration. Diabetics were shown by questionnaire to consume substantially more saccharin than non-diabetics, and the duration of regular saccharin use by diabetics was highly correlated with the duration of diabetes. There was therefore no evidence from this study that consumption of above average amounts of saccharin had led to bladder cancer in diabetics. The proporation of current smokers among diabetics was significantly less than among non-diabetics, and this may account for a low relative risk of lung cancer in the former (0-72).