High on-treatment platelet reactivity (HTPR) on clopidogrel correlates with adverse outcomes in patients treated with percutaneous coronary intervention (PCI). Whether HTPR is a modifiable risk factor for future events is not clear. We evaluated the effect of serial clopidogrel dose adjustment based on platelet function testing (PFT) during 12 months of dual antiplatelet therapy (DAPT) using Multiplate) analyzer in patients with HTPR after PCI in acute coronary syndrome on clinical outcome. Eighty-seven patients were randomized to interventional (n = 43) and control group (n = 44). Blood samples for PFT were drawn at day 1, 2, 3, 7, 30 and at month 2, 3, 6, 9 and 12. Clopidogrel dose was modified at each point of PFT in the interventional group with patients taking up to two additional 600 mg loading doses and a range of 75-300 mg maintenance dose to achieve and maintain optimal platelet reactivity (19-46 U). The incidence of the primary endpoint (composite of cardiovascular death, non-fatal myocardial infarction, target vessel revascularization and ischemic stroke) was significantly higher in the control group (36.3 vs. 16.2%; p = 0.034). There were no differences in total bleeding events (6.8 vs. 4.6%, p = ns). Patients in the interventional group maintained better P2Y12 inhibition during follow-up. We hypothesize that targeting the therapeutic window of platelet reactivity continuously throughout DAPT by dose adjustment of P2Y12 inhibitor may lead to better platelet reactivity control, and thus reduce the rate of ischemic complications in this high risk group of patients.