Objective To examine the circadian rhythms of acute myocardial infarction(AMI) in patients with type 2 diabetes. Methods From January 1994 to June 2007, 268 consecutive type 2 diabetes patients admitted to a coronary care unit with acute ST elevation myocardial infarction were studied. Another 268 age and sex matched AMI patients without type 2 diabets were taken as controls.Admission numbers were calculated at 6-hour interval within a day (circadian rhythm). The data were compared between the 2 groups.Results In diabetic group, number of patients in the first to fourth quarters was 60, 73, 64, 71 respectively, with no significant difference between different quarters.That in control group was 55, 90, 62, 61 respectively,with significant difference between the second quarter and other quarters(P<0.01). The difference between diabetic group and control group was significant. Conclusion There is significant circadian variation in the onset of AMI in the patients without diabetes,but there is no significant circadian variation in the onset of AMI in diabetic patients.