Abstract: Objective To analyze characteristics of onset time in patients with ST segment elevation myocardial infarction.Methods One thousand and twenty-three patients with STEMI were divided into different groups according to the timing of day,week and year when AMI occurred.The number and percentage of patients were observed in various subgroups.Uniformity was tested by a chi square test.Results 1) Onset was less in summer than that in other seasons.It was minimal in June(P<0.05).2) Three peaks of onset of AMI were observed in weekly distribution,they were Tuesday,Friday and Saturday.There was no significant difference between middle-aged people and old people.3) There were five peaks of onset among 24 hour in the day,they were 0～1,6～7,9～10,13～14 and 17～18 o'clock(P<0.05).Onsets were more frequent at 6～9,13～17 and 21～24 o'clock than that in other period of the day.Conclusion Onset of AMI is more frequently seen at certain times in a day,it may be related to physiological cycle and influenced by surrounding circumstances.

Key words: myocardial infarction, time of onset, season, week, circadian rhythm