Background Few data exist on the optimal duration of dual antiplatelet therapy (DAT) of aspirin and clopidogrel in patients with acute myocardial infarction (AMI) undergoing percutaneous coronary intervention (PCI). We investigated the optimal duration of DAT in the setting of AMI treated with PCI. Methods We consecutively enrolled 2376 patients who had not had any adverse events throughout 1 year following AMI and taken DAT in the COREA-AMI (COnvergent REgistry of cAtholic and chonnAm university forAMI) from January 2004 to December 2009. We divided patients into three groups according to the duration of DAT (group 1: 12-15 months, group 2: 15-21 months, group 3: longer than 21 months). The primary outcome was a composite of death, MI or stroke. Results The clinical and angiographic characteristics were similar among three groups except a larger number of patients with ST-elevation MI (STEMI) in group 3. As the duration of DAT increased, the primary outcome was significantly reduced (group 1: 9.7% vs. group 2: 6.3% vs. group 3: 4.7%, p for trend 0.012). In addition, the longer the duration of DAT, the less the cumulative rate of death or a composite of death or MI was observed (p for death and p for a composite of death or MI <0.001 in all). The occurrence of TIMI major bleeding was similar in three groups. Notably, as the duration of DAT gets longer, the relative risk was reduced 3.4 % per 1 month (hazard ratio 0.97, 95% confidence interval 0.95-0.98) in Cox proportional hazard regression model. Concluson Using real world registry, we found that longer duration of DAT more than 12 months improved clinical outcome in patients with AMI undergoing PCI.