The Health Insurance Plan of Greater New York study of coronary heart disease involves a defined population of some 110,000 adult men and women. In the first 18 months of the study, 301 men suffered an initial myocardial infarction, with a four-week case fatality rate of 32%. The focus of interest in the current report is the relationship of smoking habits and customary level of physical activity at time of infarction to both incidence and early mortality. Age-adjusted incidence rates are presented for smokers and nonsmokers of differing physical activity levels. Smokers exhibit an excess incidence rate for first myocardial infarction, but the case fatality rate is no higher than for nonsmokers. The "least active" men show both an increased incidence and a higher case fatality rate. The incidence of rapidly fatal myocardial infarction among "least active" smokers is nine times that found among the more active nonsmokers.