<h3>Importance</h3> Clopidogrel monotherapy after short dual antiplatelet therapy (DAPT) after percutaneous coronary intervention (PCI) has not yet been fully investigated in patients with acute coronary syndrome (ACS). <h3>Objective</h3> To test the hypothesis of noninferiority of 1 to 2 months of DAPT compared with 12 months of DAPT for a composite end point of cardiovascular and bleeding events in patients with ACS. <h3>Design, Setting, and Participants</h3> This multicenter, open-label, randomized clinical trial enrolled 4169 patients with ACS who underwent successful PCI using cobalt-chromium everolimus-eluting stents at 96 centers in Japan from December 2015 through June 2020. These data were analyzed from June to July 2021. <h3>Interventions</h3> Patients were randomized either to 1 to 2 months of DAPT followed by clopidogrel monotherapy (n = 2078) or to 12 months of DAPT with aspirin and clopidogrel (n = 2091). <h3>Main Outcomes and Measures</h3> The primary end point was a composite of cardiovascular (cardiovascular death, myocardial infarction [MI], any stroke, or definite stent thrombosis) or bleeding (Thrombolysis in MI major or minor bleeding) events at 12 months, with a noninferiority margin of 50% on the hazard ratio (HR) scale. The major secondary end points were cardiovascular and bleeding components of the primary end point. <h3>Results</h3> Among 4169 randomized patients, 33 withdrew consent. Of the 4136 included patients, the mean (SD) age was 66.8 (11.9) years, and 856 (21%) were women, 2324 (56%) had ST-segment elevation MI, and 826 (20%) had non–ST-segment elevation MI. A total of 4107 patients (99.3%) completed the 1-year follow-up in June 2021. One to 2 months of DAPT was not noninferior to 12 months of DAPT for the primary end point, which occurred in 65 of 2058 patients (3.2%) in the 1- to 2-month DAPT group and in 58 of 2057 patients (2.8%) in the 12-month DAPT group (absolute difference, 0.37% [95% CI, −0.68% to 1.42%]; HR, 1.14 [95% CI, 0.80-1.62];<i>P</i>for noninferiority = .06). The major secondary cardiovascular end point occurred in 56 patients (2.8%) in the 1- to 2-month DAPT group and in 38 patients (1.9%) in the 12-month DAPT group (absolute difference, 0.90% [95% CI, −0.02% to 1.82%]; HR, 1.50 [95% CI, 0.99-2.26]). The major secondary bleeding end point occurred in 11 patients (0.5%) in the 1- to 2-month DAPT group and 24 patients (1.2%) in the 12-month DAPT group (absolute difference, −0.63% [95% CI, −1.20% to −0.06%]; HR, 0.46 [95% CI, 0.23-0.94]). <h3>Conclusions and Relevance</h3> In patients with ACS with successful PCI, clopidogrel monotherapy after 1 to 2 months of DAPT failed to attest noninferiority to standard 12 months of DAPT for the net clinical benefit with a numerical increase in cardiovascular events despite reduction in bleeding events. The directionally different efficacy and safety outcomes indicate the need for further clinical trials. <h3>Trial Registration</h3> ClinicalTrials.gov Identifiers:NCT02619760andNCT03462498

Comparison of Clopidogrel Monotherapy After 1 to 2 Months of Dual Antiplatelet Therapy With 12 Months of Dual Antiplatelet Therapy in Patients With Acute Coronary Syndrome