Objective-To examine the association between smoking, alcohol consumption, and the incidence of non-insulin dependent diabetes mellitus in men of middle years and older. Design-Cohort questionnaire study of men followed up for six years from 1986. Setting-The health professionals' follow up study being conducted across the United States. Subjects-41 810 male health professionals aged 40-75 years and free of diabetes, cardiovascular disease, and cancer in 1986 and followed up for six years. Main outcome meausre-Incidence of non-insulin dependent diabetes mellitus diagnosed in the six years. Results-During 230 769 person years of follow up 509 men were newly diagnosed with diabetes. After controlling for known risk factors men who smoked 25 or more cigarettes daily had a relative risk of diabetes of 1 . 94 (95% confidence interval 1 . 25 to 3 . 03) compared with non-smokers. Men who consumed higher amounts of alcohol had a reduced risk of diabetes (P for trend <0 . 001). Compared with abstainers men who drank 30 . 0-49 . 9 g of alcohol daily had a relative risk of diabetes of 0 . 61 (95% confidence interval 0 . 44 to 0 . 91). Conclusions-Cigarette smoking may be an independent, modifiable risk factor for non-insulin dependent diabetes mellitus. Moderate alcohol consumption among healthy people may be associated with increased insulin sensitivity and a reduced risk of diabetes.