Survivors of a documented myocardial infarction are recognized as having a high risk of dying relative to the general population. Serious arrhythmias, occurring with or without evidence of new infarction, are a common cause of death in this population. Theoretically, an agent which (1) can block the sympathetic nervous activity thought to be involved in precipitating sudden death and (2) has non-neurogenic antiarrhythmic properties would be of value to people with coronary heart disease. Propranolol, like other beta- blocking agents, has these as well as other properties and therefore might be expected to prevent or retard complications of coronary heart disease such as serious arrhythmias. This would be reflected in a decrease in mortality due to coronary heart disease.