Several studies have observed an increased occurrence of acute myocardial infarction (AMI) in the morning based on subjective self-reports and objective confirmation. Evidence has also been collected to suggest a circadian variation in the onset of sudden cardiac death and silent myocardial ischemia. No published reports have examined the time of onset of AMI in relation to time after awakening. The present study examines the times of onset of AMI in relation to awakening in 137 patients with confirmed AMI. Information concerning time of awakening on the day of AMI revealed a marked increase in the onset of initial AMI symptoms within the first hour after awakening. Of the patients studied, approximately 23% reported onset of the initial symptoms of AMI within 1 hour after awakening. An increased onset of symptoms of AMI soon after awakening was also observed when patients in whom the acute cardiac symptoms were known to or may have caused awakening were excluded from consideration. This was also noted in subgroups of AMI patients classified according to age, order and location of AMI. These results extend previous observations of the circadian morning increase of AMI onset and assist in narrowing the search for potential triggers of the circadian variation of onset of AMI to physiologic changes that may occur soon after awakening.