A total of 973 patients were enrolled two to six months after hospitalization for an acute coronary event. Patients were derived from geographically dispersed centers to enhance generalization of the results to the overall population at risk. Ambulatory 24-hour Holter electrocardiographic recordings were used to evaluate the frequency, severity, and duration of asymptomatic ST depression (ASTD) as a marker of silent myocardial ischemia during usual daily activities. This information was analyzed in conjunction with ST segment depression on treadmill exercise, reversible myocardial perfusion defects on stress thallium testing, and a limited number of prespecified clinical parameters to assess the contribution which ASTD added to prognostic information from then current diagnostic techniques used to evaluate myocardial ischemia. Patients were seen by the study coordinator every four months during the first year, then every six months thereafter. At each visit the study coordinator interviewed patients to determine interim history, functional status symptoms, and drug usage. At the four month visit a second 24-hour Holter recording was obtained. At the 12-month visit and at study termination, 12-lead electrocardiograms were recorded and sent to the ECG Core Laboratory for analysis. Data analyses were used: to assess the predictive usefulness and independence of ASTD in ambulatory patients with established coronary heart disease; to evaluate the interactions and associations between the frequency, severity, and duration of ASTD and other measures of myocardial ischemia; to obtain insight into suspected vasoactive and increased myocardial oxygen consumption mechanisms of myocardial ischemia; to better understand the interrelationship between myocardial ischemia and ventricular arrhythmias as it related to subsequent cardiac mortality and morbidity.