Successful thrombolysis alters the pattern of creatine kinase (CK) release to the plasma after acute myocardial infarction (AMI). Among the important differences there are the early peak of the CK activity curve and the higher peak value for a given infarct size. To determine whether the magnitude of peak CK following thrombolysis still reflects the extent of myocardial damage, we correlated the peak CK value with left ventricular ejection fraction (LVEF) in 114 patients with first anterior AMI who had early peak CK ( ≤ 12 h) after thrombolysis. There was a significant (p < 0.001) linear relation between the peak CK value and LVEF both at admission and 2 months later. High (≧ 1,500 IU/1) as compared with low early peak CK was associated with significantly lower LVEF (p < 0.001) and a higher incidence of poor LVEF (p < 0.05).