To describe the relation between the extent of a myocardial infarct, measured according to maximum serum enzyme activity of lactate dehydrogenase, and mortality at 10 years. In 759 patients with acute myocardial infarction in whom serum activity of heat stable lactate dehydrogenase had been determined every 12 hours for 108 hours after randomisation in an early intervention trial with metoprolol. Mortality at 10 years in relation to quartile of maximum serum lactate dehydrogenase activity and history of cardiovascular disease. Among all patients mortality at 10 years was 39% in the lowest quartile, 51% in the second quartile, 50% in the third, and 59% in the fourth (p < 0.001 for relation between infarct size and 10 year mortality). Among patients without a history of myocardial infarction, angina pectoris, diabetes mellitus, or hypertension the mortality in each quartile was 29%, 32%, 41%, and 56%, respectively (p < 0.001 for relation between infarct size and 10 year mortality). Among patients with any of these risk indicators the association between the estimated infarct size and mortality at 10 years was weak (p < 0.05). Estimated size of a myocardial infarct and mortality over 10 years seem to be related but mainly in patients without a history of cardiovascular disease.