The relationship between acute myocardial infarct (AMI) size and morbidity and mortality was estimated in 317 patients followed for one year or until death. Infarct size was estimated from serum creatine kinase (CK)-MB levels measured thrice daily. The incidence of ventricular arrhythmias, congestive heart failure, cardiogenic shock, and the cardiac performance during exercise were studied during hospitalization. Hospital mortality and one-year mortality were registered. A positive correlation was found between serum CK-MB-estimated infarct size and the incidence of ventricular arrhythmias (p less than 0.05). Patients with congestive heart failure and patients with cardiogenic shock had significantly larger infarct size than patients without (p less than 0.05-0.01), although there was a substantial overlap. During exercise test the rise in systolic blood pressure correlated negatively and the rise in heart rate correlated positively to estimated infarct size (p less than 0.01). Both hospital mortality and one-year mortality were significantly related to estimated infarct size (p less than 0.01). Thus the infarct size, as estimated from serum CK-MB, seems to be of importance for development of the most common and serious complications after AMI.