Background—- Previous studies have shown that there is a circadian variation in the incidence of acute myocardial infarction. The aim of this study is to examine the circadian rhythms of acute myocardial infarction in patients with type 2 diabetes. Methods and Results—1016 consecutive patients admitted to a coronary care unit with acute ST elevation myocardial infarction were studied from January 2004 to December 2008. All patients were divided into two groups according to with or without diabetes. Admission rates were calculated according to the 6-hour interval of the day (circadian rhythm). The data were analyzed for variations within subgroups. In diabetic group, number of patients in the first to fourth quarters was 38, 45, 43, 46 respectively (NS). The corresponding figures for the controls were 174, 295, 183, 192 (P<0.01). The difference between the two groups was significant (P<0.02). Conclusion—There is no a significant circadian variation in the onset of acute myocardial infarction in diabetic subjects.