In this study of 3,000 males the statistical association of coronary artherosclerotic heart disease (CAHD) and smoking habits was most striking under the age of 51. In this group 21 (11.7%) of 179 heavy cigarette smokers had CAHD, whereas 100 (6.5%) of 1,521 other individuals had CAHD. The ratio of myocardial infarcts to isolated angina pectoris was 1.3:1 in heavy smokers, and 0.5:1 in other categories. Heavy cigarette smoking was also associated with higher serum cholesterol levels. It was suggested not so much as a factor in atherogenesis, but as a factor in precipitating acute clinical events in individuals with advanced CAHD. Cigar, pipe, and light cigarette smoking did not appear to be related to the problem.