Myocardial infarction (MI) usually results from thrombotic coronary artery occlusion at the site of a ruptured atherosclerotic plaque. The factors responsible for triggering MI are not known but conditions that increase serum catecholamines may be involved. Accordingly, the authors prospectively evaluated the presence of factors that may increase catecholamines immediately prior to MI in 186 patients. Myocardial infarction was documented by a rise in serum CK-MB. There were 149 men and 37 women, aged 57 ± 12 (mean ± SD) years. All patients were interviewed within 72 hours of admission concerning strenuous physical activity, emotional stress, and assumption of the upright posture immediately prior to the onset of symptoms. Seventy-five (40%) patients had one or more of these factors immediately prior to the onset of MI chest pain. Fourteen (8%) experienced acute emotional upset; 28 (15%) were involved in strenuous physical activity; and 39 (21%) had suddenly changed position. In the latter group, 18 changed from being supine for more than 1 hour to standing; three rose from supine to sitting; and 18 changed from prolonged sitting (> 60 minutes) to standing. The conclusion is that a potential triggering factor is present in many patients immediately prior to the onset of MI. A sudden change in position is the most frequent potential trigger, typically occurring the morning after awakening from sleep.