The events that convert chronic coronary artery disease to acute myocardial infarction or sudden cardiac death are poorly understood. We studied the time of day of onset of nonfatal myocardial infarction (as judged objectively by CK-MB values, n= 703) and sudden cardiac death (as judged by death certificates, n= 2,203) to determine if there was an increased incidence associated with a particular time of day. Both conditions were found to have prominent circadian rhythms with a primary peak in the morning and a secondary peak in the evening. Platelet aggregability, which has been proposed as a component of myocardial infarction and sudden cardiac death, was also found to increase during the morning. These findings suggest a mechanism for myocardial infarction and sudden cardiac death and may lead to improved methods of prevention.