Multiple studies have shown a correlation between high on-treatment platelet reactivity (HPR) and ischemic complications after percutaneous coronary interventions (PCI); however, the role of platelet reactivity testing in order to adjust clopidogrel dose is debated. We sought to determine whether a strategy incorporating platelet reactivity testing with the Multiplate analyzer to tailor the dose of clopidogrel is superior to standard clopidogrel treatment after PCI. Between May 2008 and June 2009, 192 consecutive patients undergoing PCI were randomized to a tailored treatment strategy using the Multiplate analyzer or to uniform administration of 75 mg clopidogrel. In the tailored group, platelet function was assessed 24 h after clopidogrel loading, and patients with HPR (>46 U) received an additional 600 mg loading dose and 150 mg clopidogrel thereafter for one month. The primary endpoint was the composite of cardiac death, myocardial infarction, ischemic stroke or definite/probable stent thrombosis during six months. In the tailored group, a repeated loading dose of 600 mg clopidogrel significantly decreased platelet reactivity in patients with HPR (61.0 U [IQR: 52.5-71.5] vs. 21.5 U [15.8-30.5]; P < 0.0001) that remained unchanged during the maintenance phase on 150 mg clopidogrel (25.0 U [IQR: 19.8-27.0]; P = 0.20). The incidence of the primary endpoint was significantly higher in the standard clopidogrel group as compared to the Multiplate-tailored arm (5.3% vs. 0%, P = 0.03). In parallel, MACCE-free survival significantly improved in patients with Multiplate-tailored therapy (Kaplan-Meier log-rank: P = 0.02). Increasing the dose of clopidogrel according to the Multiplate assay may reduce ischemic complications in patients on clopidogrel after PCI.