Background: Triple anti-platelet therapy (TA) has been reported to be superior to dual anti-platelet therapy in acute myocardial infarction (AMI) patients. However, the optimal TA duration remains unclear. Methods and Results: We retrospectively analyzed 716 patients with AMI undergoing DES implantation between Nov 2005 and May 2008. We excluded patients who had suffered major adverse cardiac events (MACE) for 3 months after index procedures. Patients were assigned in groups based on TA duration as follows: 1 to 30 days (n=265), 1 to 3 months (n=232), 3 to 6 months (n=86) and more than 6 months (n=133). We compared the incidence of 1-year MACE [such as composite of cardiac death, non-fatal AMI, stent thrombosis and target vessel revascularization (TVR)] between groups. The median durations of TA in each group were 18.15 짹 7.26 days, 52.31 짹 15.55 days, 129.21 짹 25.44 days and 317.02 짹 75.38 days, respectively. The mean age was higher in 1 to 3 months duration group than those of the other groups (62.18 짹 11.37 vs. 63.17 짹 11.27 vs. 60.84 짹 13.34 vs. 59.56 짹 11.68 years, p=0.030). However, there were no significant differences in the other baseline characteristics between groups. There were no significant difference of incidences of cardiac death, non-fatal AMI, stent thrombosis, TVR and MACE at 1-year follow-up between groups (2.6% vs. 3.4% vs. 1.2% vs. 0.8%, p=0.349; 0.4% vs. 1.3% vs. 0.0% vs. 0.0%, p=0.303; 0.4% vs. 1.3% vs. 1.2% vs. 0.0%, p=0.789; 6.0% vs. 9.1% vs. 4.7% vs. 5.3%, p=0.354 and 9.1% vs. 12.5% vs. 5.8% vs. 6.0%, p=0.121). The rates of bleeding complications were not different between groups. However, the TA duration group of more than 3 months had lower incidence of the composite of cardiac death, non-fatal AMI and stent thrombosis and MACE than those of less than 3 months (3.8% vs. 0.9%, p=0.032; 10.7% vs. 5.9%, p=0.044). By Cox regression analysis, Killip class III and IV and the TA duration group of more than 3 months were independent predictor of 1-year MACE [Harzad ratio(HR)=2.422; 95% Confidence interval (CI)=1.062-5.522, HR=3.583; 95% CI=1.276-10.058 and HR=0.529; 95% CI=0.281-0.998]. Conclusion: Our data shows that longer duration of TA isn���t always associated with better clinical outcomes. However, TA duration of more than at least 3 months appears more effective in patients with AMI undergoing DES implantation.