Introduction Clopidogrel is an antiplatelet agent that is widely used for the secondary prevention of cardiovascular and cerebrovascular events. The genotype of cytochrome P450 2C19 (CYP2C19) differentially affects the liver's metabolism of clopidogrel, which may influence the drug's response and efficacy for cardiovascular event prevention. In contrast to prior studies of patients with coronary artery diseases, little is known about whether the CYP2C19 genotype influences the preventive efficacy of clopidogrel in patients who had a stroke. We hypothesise that, among patients who had an acute ischaemic stroke who are prescribed clopidogrel, the patients with a loss-of-function CYP2C19 genotype (poor and intermediate metabolisers) may be at a higher risk of composite cardiovascular events than those who are non-carriers (extensive metabolisers). Methods and analysis This prospective observational multicentre study was designed to determine whether composite cardiovascular events would differ among patients who had an ischaemic stroke prescribed clopidogrel according to CYP2C19 genotype (poor or intermediate vs extensive metabolisers). Inclusion criteria were patients who had an acute ischaemic stroke who underwent CYP2C19 genotype evaluation and received clopidogrel within 72 hours of stroke onset. The primary outcome is composite cardiovascular events (stroke, myocardial infarction, or cardiovascular death) within 6 months after acute ischaemic stroke between patients categorised as poor or intermediate metabolisers and those categorised as extensive metabolisers according to their CYP2C19 genotype. Ethics and dissemination The Institutional Review Board of Severance Hospital, Yonsei University College of Medicine approved this study (3-2019-0195). We received study approval from the institutional review board of each participating hospital. We plan to disseminate our findings at relevant conferences and meetings and through peer-reviewed journals. Trial registration number NCT04072705.