OBJECTIVE The aim was to investigate the circadian and weekly variation in Chinese young patients with acute myocardial infarction (AMI). METHODS This was a 10-year retrospective cohort study. We studied patients (>18 to <45 years of age) with a first attack of AMI from the emergency departments of 3 university teaching hospitals in Taiwan from January 1, 2001, to December 31, 2010. We analyzed patients in the standard circadian fashion using 6-hour intervals (00:01-06:00, 06:01-12:00, 12:01-18:00, and 18:01-24:00). We also did an analysis by day of week. RESULTS The database had 505 patients with AMI with complete data. The percentage of total AMIs that occurred in the 6-hour intervals were as follows: 00:01 to 06:00, 30.9%; 06:01 to 12:00, 23.4%; 12:01 to 18:00, 25.9%; and 18:01 to 24:00, 19.8%. The percentage of AMIs between 00:01 and 06:00 was significant higher compared with that in the other three 6-hour intervals (df = 3, χ(2) = 91.7, P < .001). However, there was no significant weekly variation for these patients in the present study. CONCLUSIONS There was a significant circadian variation with a peak from 00:01 to 06:00 in Chinese young patients with AMI. However, there was no significant weekly variation in these patients. The circadian periodicity may create new possibilities for disease prevention and medication prescription.