Circadian variation of the onset of acute myocardial infarction has been noted in many studies and may carry important pathophysiologic implications. However, only a few previous studies have attempted subgroup analyses. In 4,796 patients with documented acute myocardial infarction, the time of symptom onset was recorded. As in other studies, the peak of onset occurred in the morning from 6:01 AM to 12:00 noon, and 28% of the population (1.16 times the average percentage for the other time periods) experienced symptom onset in that period (p less than 0.001). There was a second, lower peak (25%) in the evening between 6:01 PM and 12:00 midnight, which was also observed in some previous studies. We sought to determine whether or not the presence of subgroups with specific clinical characteristics would exhibit different patterns and thereby contribute to these peaks in the overall population. In patients with a history of congestive heart failure (n = 606) or with non-Q wave infarction (n = 832), a pronounced peak (29%) occurred only in the evening. Two nearly equal peaks were observed in patients older than 70 years of age (n = 1,422), smokers (n = 2,057), diabetics (n = 767), women (n = 1,213), and patients taking beta-blocking drugs (n = 847). Finally, in patients with a previous myocardial infarction (n = 1,104), no peaks were observed.(ABSTRACT TRUNCATED AT 250 WORDS)