To evaluate the effect of age on circadian and seasonal variation in the onset of acute myocardial infarction (AMI) in Japan, we studied 725 patients whose onset date and time of the symptom were determined. In the analysis of all AMI patients, the occurrence showed significant circadian variation (P< 0.01); 30% of episodes occurred between 0600 and 1200h which formed a dominant morning peak. In the analysis of separate age-groups, a significant difference in the occurrence among four 6-hour periods of day was observed only in the early-elderly group (65-74 years), in which a significant (P < 0.01) morning peak comprising 33% of episodes was observed between 0600 and 1200 h. The monthly variation of AMI onset in total subjects tended to show two peaks; April and August. The seasonal variation tended to show a trough in autumn when analyzed in all patients, but the variation was significant only in middle-aged group (< 65 years). In conclusion, the incidence of AMI in Japanese population showed dominant morning peak, and tended to show autumn trough. These circadian and seasonal variations were affected by age.