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Background: All guidelines recommend 12-month dual antiplatelet therapy (DAPT) for patients (pts) with acute coronary syndrome (ACS). In Asia, little information is available about the contemporary real world data for DAPT duration and its impact on clinical outcomes in ACS pts treated with percutaneous coronary intervention (PCI). Methods and Results: The Taiwan ACS Stent Registry was a prospective, multicenter, cohort study to observe clopidogrel-based DAPT in ACS pts treated with PCI from 2012 to 2015. The clinical outcomes in relation to DAPT duration (< 6, ≥ 6 to ≤ 9, and > 9 months [mos]) were compared. The primary outcome was the composite of cardiovascular death, myocardial infarction (MI) and stroke. Stent thrombosis was the secondary endpoint. Overall, 2,222 ACS pts (62 yrs, 83% men) were included. Among them, 55% were ST-elevation MI, 36% received > 1 stent and 51% used drug-eluting stents. Overall, the mean duration of DAPT was 263±120 days. DAPT duration was < 6 mos in 23.9% of pts; 6 to 9 mos in 17.5%; and > 9 mos in 58.6%. At 1 yr follow-up, only 35.6% pts were treated with DAPT. The primary outcome occurred in 3.2%, 2.6% and 1.3% (p = 0.020) in pts with DAPT duration < 6, 6 to 9, and > 9 mos at 1 yr follow-up. Stent thrombosis occurred in 0.5% pts and there was no difference between the DAPT duration groups. Propensity score adjusted analysis was performed. Compared with < 6 mos, DAPT duration > 9 mos was associated with a lower risk of primary outcome (odds ratio [OR] 0.479, 95% confidence interval [CI] 0.238, 0.963), but not 6 to 9 mos (OR 0.952, 95% CI 0.425, 2.135). Multivariate logistic regression analysis showed that pts with diabetes, atrial fibrillation or received bare metal stent were more likely to have DAPT < 6 mos. The most common reason (72%) to stop clopidogrel before 12 mos was due to the Taiwan National Health Insurance reimbursement regulations which only cover 9 mos P2Y12 inhibitor after ACS. Conclusions: Only 35.6% pts with ACS treated with PCI received DAPT for 12 mos in Taiwan. DAPT ≤ 9 mos increased the risk of primary outcome. The reimbursement regulation is the most common cause to stop clopidogrel before 12 mos. [ABSTRACT FROM AUTHOR]