Seasonal and circadian variations in the occurrence of myocardial infarction and sudden cardiac death have been documented, suggesting that triggering factors may play a role in the causation of cardiac events. However, there are only sparse and conflicting data on the weekly distribution of the disorders.Methods and Resultsand Results To determine the weekly variation of acute myocardial infarction and sudden cardiac death, 5596 consecutive patients (71% men; age, 63±1 years) were analyzed in a regionally defined population (n=330 000; age, 25 to 74 years) monitored from 1985 to 1990. The exact time of onset of symptoms was used to determine the day of the event. Patients with myocardial infarction (n=2636) demonstrated a significant weekly variation (P< .01) with a peak on Monday, whereas patients with sudden cardiac death (n=2960) were evenly distributed throughout the week. A similar weekly pattern was observed in subgroups of patients with myocardial infarction defined with respect to age, sex, cardiac risk factors, prior cardiac medication, and infarct characteristics. The working population demonstrated a weekly variation of myocardial infarction as opposed to the nonworking population, with a 33% increase in relative risk of disease onset on Monday (P< .05) and a trough on Sunday compared with the expected number of cases, if homogeneity was assumed.ConclusionsThe onset of acute myocardial infarction demonstrates a peak on Monday primarily in the working population. If this finding is confirmed in other communities, it may aid in identifying acute triggering events of myocardial infarction and perhaps in improving prevention of the disease.