The whole blood clotting time, plasma fibrinogen and individual coagulation factors II, V, VII, VIII, IX, X, XI and XII were serially measured in 14 patients over a 10-day period following acute myocardial infarction. A further six patients with similar symptoms but without evidence of myocardial infarction were also studied. The whole blood clotting time was significantly shorter within the first 24 h after infarction than on subsequent days. There were significant increases in the levels of fibrinogen and of factors VIII, IX and XI and a significant decrease in factor XII in the patients who had sustained a myocardial infarction. The patients without myocardial infarction had no significant change in any of the coagulation factors measured.