The authors examined the relation between cigarette smoking and suicide by conducting a cohort study of 300,000 male US Army personnel followed prospectively from January 1987 through December 1996 for 961,657 person-years. They found that the risk of suicide increased significantly with the number of cigarettes smoked daily (p for trend < 0.001). In multivariable-adjusted analyses, smokers of more than 20 cigarettes a day, compared with never smokers, were more than twice as likely to commit suicide. For male active-duty army personnel, the dose-related association between smoking and suicide was not entirely explained by the greater tendency of smokers to be White, drink heavily, have less education, and exercise less often.