The relationship of smoking habit to total mortality and to the incidence of new manifestations of coronary heart disease (CHD) has been examined in 2,282 middle-aged men under medical surveillance for ten years in Framingham, Mass, and 1,838 middle-aged men followed for eight years in Albany, NY. It was found that in men who report habitual consumption of 20 or more cigarettes per day the risk of myocardial infarction was about three times greater than in nonsmokers, former cigarette smokers, or pipe and cigar smokers. No relationship was shown between smoking habit and angina pectoris when this symptom was the sole manifestation of CHD. The association between heavy cigarette smoking and increased morbidity and mortality from CHD was unexplained, although it appeared to be relatively immediate. It is inferred that stopping cigarette smoking lessens the risk of CHD.