BACKGROUND: A recent clinical trial demonstrated that optical frequency domain imaging (OFDI) guidance in percutaneous coronary intervention (PCI) is noninferior to intravascular ultrasound (IVUS) guidance in patients with coronary artery disease with regard to target vessel failure (composed of cardiac death, myocardial infarction attributed to the target vessel, and clinically-driven target vessel revascularization) at 12 months. The impact of OFDI guidance in PCI for patients with acute coronary syndrome (ACS) remains uncertain. METHODS: OPINION ACS is a multicenter, prospective, randomized, controlled, open-label, parallel group, non-inferiority trial in Japan. Eligible patients will be randomly assigned to receive either OFDI- or IVUS-guided PCI. PCI is performed using the sirolimus-eluting stent in accordance with certain OFDI and IVUS criteria for optimal stent deployment. All patients will undergo follow-up angiography and OFDI imaging at 8 months. The primary endpoint is the minimum lumen area, as measured by OFDI at 8 months. CONCLUSION: The OPINION ACS trial outcomes will provide insights regarding the impact of OFDI-guided PCI on in-stent restenosis at 8 months in patients with ACS.