Background Seasonal variation with winter preponderance of myocardial infarction incidence has been described decades ago, but only a few small studies have classified myocardial infarction based on ST-segment elevation. It is unclear whether seasonal and circadian variations are equally present in warmer and colder regions. We investigated whether seasonal and circadian variations in acute myocardial infarction (AMI) are more prominent in colder northern states compared with warmer southern states. We also investigated the peak time of admission to better understand the circadian rhythm. Methods Data from the GWTG-CAD database were used. We analyzed 82,971 consecutive acute myocardial infarction (AMI) patients treated at 276 US centers from 2003 to 2008. The country was geographically divided into warmer southern and colder northern states using latitude 35 degrees for this purpose. Results Overall, acute myocardial infarction (AMI) admissions varied across seasons (P <.01), and were higher in winter (winter vs. spring n = 21,483 vs. 20,291, respectively). When stratified based on type of AMI, non–ST-segment elevation myocardial infarction (NSTEMI) admissions varied across seasons (P <.01) and were highest in winter and lowest in spring. Seasonal variation was not significant in STEMI admissions (P =.30). Seasonal variation with winter predominance was noted in AMI patients in warmer southern states (P <.01), but not in colder states. The distributions of length of stay for AMI patients and door to balloon times for STEMI patients were minimally different across all four seasons (P <.01) with longest occurring in winter. Most patients with AMI presented during daytime with a peak close to 11 am and a nadir at approximately 4 am. Conclusions Seasonal variation with winter predominance exists in AMI admissions and was significant in NSTEMI admissions but not in STEMI admissions. Seasonal variation was only significant in warmer southern states.