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: 41810 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 measure: 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. Key messages Epidemiological studies have not adequately examined the independent associations between smoking, alcohol, and the risk of diabetes after accounting for obesity This paper shows that current smoking roughly doubles the risk of diabetes among a healthy population of men Moderate alcohol consumption, however, significantly decreases the risk of diabetes Smoking and alcohol may alter the risk of diabetes through long term effects on insulin secretion and insulin resistance.