Recent reports suggest that the hematocrit value may be a significant factor in predisposition to coronary thrombosis.1-4 Hematocrit values of both men and women with myocardial infarction were observed to be higher on the average than those of age-matched control subjects. The data led to the proposal that the red cell volume of patients with coronary artery disease should be reduced by venesection in an effort to prevent thrombosis. An editorial in the Journal of the American Medical Association5 notes that these "stirring reports... tend to pick up the threads of medieval medicine...", and raise the question whether phlebotomy may be "an ancient procedure turning modern." In an independent study of the hematocrit values of normal men and of men with acute myocardial infarction we were unable to corroborate the earlier reports and found no correlation between hematocrit value and occurrence of coronary thrombosis. Methods Four groups of