Fibrinogen has been reported to be a risk factor for development of myocardial infarction in patients with ischaemic heart disease. The functional properties of plasma high molecular weight (HMW) fibrinogen would suggest that this fibrinogen species is responsible for the association. In a prospective study with a follow-up period of 6 years the plasma concentrations of clottable fibrinogen and of HMW fibrinogen were related to the subsequent development of myocardial infarction in 53 patients admitted with ischaemic heart disease. Neither the concentrations of clottable fibrinogen nor of HMW fibrinogen in the 25 patients developing myocardial infarction deviated from the concentrations in the 28 patients with an uncomplicated course. A significant positive correlation between the concentrations of clottable fibrinogen and of HMW fibrinogen (rs = 0.58, p less than 0.001) suggests that HMW fibrinogen represents a major fraction of the clottable fibrinogen concentration in patients with ischaemic heart disease.