patients who were screened but excluded. Among the 1,167 patients, no diurnal pattern of onset could be found at either 2-or 6-hour intervals. Among the 540 patients enrolled in the trial, no pattern could be found at these intervals either, although at 8-hour intervals, 27% of infarctions occurred between midnight and 8:00 AM, compared with 37% between 8:00 AM and 4:00 PM and 36% between 4:00 PM and 12:00 AM (p=0.02). In contrast to the patterns previously noted for Q wave myocardial infarction, there was no preponderance of non-Q wave infarction in the late morning. Circadian rhythm was also absent among patients not treated with P-blockers as well as among patients presenting with ST segment elevation on their enrollment electrocardiograms. Diabetics, women, and patients with first infarction were more likely to present during the afternoon hours. We conclude that the late morning preponderance seen for Q-wave myocardial infarction is not discernable in patients with non-Q wave myocardial infarction. This observation suggests that the pathogenesis of these two infarct subtypes is different or that the process of thrombotic coronary occlusion in Q wave infarction (sustained) differs from that in non-Q wave infarction (nonsustained). (Circulation 1990;81:548-555) Several recent studies1-3 have demonstrated significant diurnal variation in the onset of acute myocardial infarction. These findings confirm those of epidemiologic studies published in the 1960s and 1970s4-7 that showed that infarction is more likely to occur in the morning hours and less so during the night. The time course of these fluctuations parallels recently observed increases in platelet aggregability and increased plasma activity of various components of the clotting system,8-11 surges in adrenergic activity,10-12 as well as troughs in the activity of the intrinsic fibrinolytic system.1314 These observations were made principally in patients with Q wave infarction. In these patients, coronary angiography has shown the underlying pathology to be a From the Section

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