Background: Postconditioning in myocardial infarction showed infarct size reduction. However, there were controversies to the method of postconditioning in STEMI. The aim of our investigation was to evaluate the effects of the number and time duration of balloon inflations during primary PCI in terms of infarct size reduction. METHODS and Results: Patients who underwent primary PCI for STEMI from November 2005 to May 2008 were enrolled (n=305, 64.1짹12.4 years, 220 male). We excluded patients who experienced cardiogenic shock, ventricular tachyarrhythmia, or cardiopulmonary resuscitation before or during PCI. The major adverse cardiac events (MACE) (including death, myocardial infarction, target lesion revascularization) at 2-year (median follow-up period 3.1 years) and the correlations of peak CK, CK-MB, and LV function with number (5.9짹4.4) and total duration of balloon inflations (67.8짹58.3 sec) were analyzed. There were no significant difference in 2-year MACE according to the number and total duration of balloon inflations. However, there were negative correlation between cardiac enzyme and both number of balloon inflations [peak CK vs number (r = -0.156), peak CK-MB vs number (r = -0.143), p=0.007, p=0.013, respectively] and total duration of balloon inflations [peak CK vs total duration (r = -0.156), peak CK-MB vs total duration (r = -0.140), p=0.006, p=0.014, respectively]. Post hoc analysis showed both peak CK and peak CK-MB in patients with more than 6-time inflations and total 68 sec balloon inflations were significantly lower [peak CK 2050.0짹1918.5 vs 2579.4짹1880.6 (p=0.016), peak CK-MB 122.2짹125.8 vs 150.7짹128.5 (p=0.05)]. CONCLUSIONS: Multiple balloon inflations combined with longer duration inflations during primary PCI showed reduction of infarcted myocardium. Our data suggest that postconditioning would be used in real-world clinical practice to decrease the size of myocardial infarction. Future prospective large study are needed to clarify our results.