The serum cardiac enzymes in 30 cases of patients with acute myocardiac infarction(AMI) were studied. From the onset to 48 hours of AMI, the sera were collected in three different stages. By comparing with electrocardiogram respectively, the activities of AST,CK, LDH and CK-MB were measured. The results revealed that the highest positive rate of serum cardiac enzymes were 90%(AST,CK-MB), and the lowest positive rate was 70%(LDH).By using Q wave as a confirmatory index of AMI, the negative rate of cardiac enzymes was 40%. The early detection of increasing cardiac enzymes could indicate the presence of AMI. An increasing of CK-MB in patients with AMI indicated a possible new infarction occurred. A patient had increasing of cardiac enzymes within 6 hours of AMI might have a poor prognosis.