Background Although ischemia induced QRS complex changes have been described previously, their relationship with impaired reperfusion, which is known to be associated with poor clinical outcomes, is not clear. Methods and Results To investigate the relationship of QRS duration changes with myocardial reperfusion, we studied 165 acute myocardial infarction (MI) patients who were administered fibrinolytic therapy for reperfusion. For each patient, 12-lead electrocardiogram (ECG) with a paper speed of 50 mm/s was recorded on admission and repeated at the 60^th and 90^th min following fibrinolytic therapy. Based on the myocardial blush grades obtained from a control coronary angiography, patients were divided into reperfusion (grades 2, 3) and impaired reperfusion (grades 0, 1) groups. We detected impaired reperfusion in 74 patients. The patients in the impaired reperfusion group were older, more often diabetic, and had longer pain-to-needle intervals. They also had significantly longer QRS durations at admission compared to reperfusion group patients (91±11 vs 79±11 ms, p<0.001). Reperfusion group patients showed significantly greater resolution in QRS duration at the 90^th min post fibrinolysis ECG (18±5 vs 5±4 ms, p<0.001). In regression analysis, the pain-to-needle time (odds ratio (OR): 0.55, 95% confidence interval (CI) 0.35-0.88, p=0.012), QRS duration on admission (OR: 0.80, 95% CI 0.67-0.97, p=0.021), and change in QRS duration at the post fibrinolysis 90^th min ECG (OR: 2.43, 95% CI, 1.29-4.60, p=0.006) were determined as independent predictors of adequate reperfusion. Conclusion The present study, for the first time, has shown that admission QRS duration and post fibrinolysis 90^th min QRS duration changes are related to tissue reperfusion in fibrinolytic administered acute MI patients. (Circ J 2008; 72: 873 - 879)