Sixteen patients with acute myocardial infarction were studied with continuous cardiac monitoring for 1 to 3 weeks. Patients were excluded from the study if arrhythmias were present on admission or if arrhythmias might have occurred as a result of treatment. The results indicate that cardiac arrhythmias occur with a much higher frequency (94 per cent) than previously reported. They may occur many days after the infarction. They occur in asymptomatic patients and may go undetected by conventional observation. Continuous cardiac monitoring has immediate practicality in the management of these patients, and might, if used as a guide to therapy,[see table in the PDF file]reduce the mortality from acute myocardial infarction.