The circadian, day-of-week, and seasonal distributions of acute myocardial infarction and its association with the patients' working status were analyzed in a regionally defined population (n = 103, 322) monitored from 1980 to 1988. Included were 2906 consecutive patients (1746 [60.1%] men and 1160 [39.9%] women; mean age 67.8 years) with confirmed diagnosis (by standardized diagnostic criteria in hospitalized patients and autopsy results in out-of-hospital deaths). The time of myocardial infarction on the basis of onset of symptoms was known in 1901 cases. Myocardial infarction occurred more frequently (p < 0.05) during the morning from 7:00 to 10:00 AM, on Mondays, and during the winter from January to March compared with other times of day, days of the week, and seasons. Compared with retired patients, working patients (32%) had a second circadian peak, in the afternoon at 4:00 PM (p < 0.05), and a trend toward an additional seasonal peak in September (p value not significant), whereas the day-of-week pattern was similar in the two subgroups. The occurrence of myocardial infarction demonstrates marked circadian, day-of-week, and seasonal variations, with some differences between working and retired patients. Further investigation of possible triggering events may aid in identifying underlaying mechanisms and perhaps in improving prevention of the disease.