Background: The guideline-recommended low-density lipoprotein cholesterol target level of <70 mg/dL may not be achieved with statin administration in some patients with acute coronary syndrome (ACS). Therefore, the proprotein convertase subtilisin-kexin type 9 (PCSK9) antibody can be added to high-risk patients with ACS. Nevertheless, the optimal duration of PCSK9 antibody administration remains unclear. Methods and Results: Patients were randomized to receive either 3 months of lipid lowering therapy (LLT) with the PCSK9 antibody followed by conventional LLT (with-PCSK9-antibody group) or 12 months of conventional LLT alone (without-PCSK9-antibody group). The primary endpoint was the composite of all-cause death, myocardial infarction, stroke, unstable angina, and ischemia-driven revascularization. A total of 124 patients treated with percutaneous coronary intervention (PCI) were randomly assigned to the two groups (n = 62 in each). The primary composite outcome occurred in 9.7% and 14.5% of the patients in the with- and without-PCSK9-antibody groups, respectively (hazard ratio: 0.70; 95% confidence interval: 0.25 to 1.97;p= 0.498). The two groups showed no significant differences in hospitalization for worsening heart failure and adverse events. Conclusions: In ACS patients who underwent PCI, short-term PCSK9 antibody therapy with conventional LLT was feasible in this pilot clinical trial. Long-term follow-up in a larger scale clinical trial is warranted.