The Myocardial Infarction Onset Study (Determinants Study) supported from 1989 to 1993 had a case-control design. Coronary care units in the Boston and Worcester, Massachusetts areas collaborated. Eligible patients were interviewed concerning: whether the myocardial infarction was the first or not and the nature and timing of pain; the time of waking the day of onset of the infarction and the time of assuming upright posture; the number of times the patient awoke from sleep and assumed the upright posture during the night or day prior to myocardial infarction; whether the patient was receiving therapy with a beta-blocker or aspirin and at what time the medication was taken in the 24 hours preceding the myocardial infarction; and age and smoking status. A questionnaire was administered on: educational background; marital status; occupation; usual activity in recent past and, specifically, on the day before onset including diet, coffee consumption, smoking, and exercise; sleep habits; geographic location where the infarct occurred. Data were also abstracted from the medical record. Patient enrollment ended in the fourth year. Relative risks associated with hypothesized risk factors for onset of myocardial infarction were estimated by comparing observed exposure frequencies immediately prior to myocardial infarction with expected values derived from exposure frequencies in the same patients at other times of the day and in other patients at the same time of day.