Mean platelet volume and count were measured in three groups: patients with acute myocardial infarction, a control group with myocardial ischaemia but no infarction and an asymptomatic group of young males. Mean platelet volume was significantly larger in the myocardial infarction group compared with the ischaemic heart disease group or the asymptomatic group. Two subpopulations were present within the myocardial infarction group. One subgroup had a large mean platelet volume and low count. The other subpopulation was indistinguishable, with regard to platelet count and mean volume, from the ischaemic heart disease group. Over 60% of the myocardial infarction group lay in the area of high platelet volume and low count compared with 13% of the ischaemic heart disease control group and 38% of the asymptomatic group. Acute myocardial infarction is likely to be associated with a large mean platelet volume and low count compared with the ischaemic heart disease group. There is no statistical evidence that this condition is related to smoking or size and site of infarct. This evidence suggests that large mean platelet volume and low platelet count could be a major risk factor for myocardial infarction.