Abstract The multiphasic-examination findings of 464 persons in whom a first myocardial infarction later developed were compared with those of two control groups, one matched for age, sex and race (ordinary controls), and the other matched in addition for standard coronary risk factors (risk controls). The total leukocyte count, measured, on the average, 16.8 months before the myocardial infarction, was strikingly related to development of infarction. The mean leukocyte count in cases was significantly higher (p<0.001) than in either control group. Ascending from lowest to highest quartile in the cases and ordinary controls the increase in risk of myocardial infarction associated with the leukocyte count was similar to that found for cholesterol and blood pressure. Cigarette smoking, which was strongly related to the leukocyte count, may account for about two thirds of the relation of the count to infarction. The leukocyte count may prove valuable in the routine assessment of risk of myocardial infarction...