We examined the effect of light smoking in relation to incidence of coronary heart disease (CHD) in a general population sample consisting of 6879 men aged 47-55 years and free of previous myocardial infarction (MI) at baseline. After a follow-up of mean duration 11.8 years, 11.0% of men smoking 1-4 cigarettes daily (n = 228) had suffered a major CHD event, compared to 3.7% of non-smokers (n = 2049) [adjusted odds ratio 2.8 (1.7-4.7)]. No further increase in risk was observed in men who smoked more. There was an increasing risk of death from cancer with the number of cigarettes smoked per day. Mortality was increased in all categories of cigarette smokers, particularly among the very heavy smokers, who had a mortality risk of 22% compared to 6% among nonsmokers [adjusted odds ratio 4.4 (2.7-7.1)]. Data from an examination 4 years later considered only those men who stated that their smoking habits were identical on both occasions (n = 3981). Among these subjects the incidence of CHD after a mean period of 7.1 years was 10.6% in men smoking 1-4 cigarettes per day, compared to 2.6% in nonsmokers [adjusted O.R. 4.6 (2.1-10.1)]. No dose-response effect was observed. Even very light cigarette smoking considerably increases the risk of CHD in middle-aged men.