Compared with the approved dose regimen of clopidogrel (300-mg loading dose [LD], 75-mg maintenance dose [MD]), prasugrel has been demonstrated to reduce ischaemic events in patients with acute coronary syndrome (ACS) undergoing percutaneous coronary intervention (PCI). In ACS, antiplatelet effects of a prasugrel MD regimen have not been previously compared with either a higher clopidogrel MD or after switching from a higher clopidogrel LD. The objective of this study was to evaluate the antiplatelet effect of a prasugrel 10-mg MD versus a clopidogrel 150-mg MD in patients with ACS who had received a clopidogrel 900-mg LD. Patients with non-ST elevation ACS, treated with aspirin and a clopidogrel 900-mg LD, were randomised within 24 hours post-LD to receive a prasugrel 10-mg or clopidogrel 150-mg MD. After 14 days of the initial MD, subjects switched to the alternative treatment for 14 days. The primary endpoint compared maximum platelet aggregation (MPA, 20 microM adenosine diphosphate [ADP]) between prasugrel and clopidogrel MDs for both periods. Responder analyses between treatments were performed using several platelet-function methods. Of 56 randomised subjects, 37 underwent PCI. MPA was 26.2% for prasugrel 10 mg and 39.1% for clopidogrel 150 mg (p<0.001). The prasugrel MD regimen reduced MPA from the post-900-mg LD level (41.2% to 29.1%, p=0.003). Poor response ranged from 0% to 6% for prasugrel 10 mg and 4% to 34% for clopidogrel 150 mg. Thus, in ACS patients a prasugrel 10-mg MD regimen resulted in significantly greater platelet inhibition than clopidogrel at twice its approved MD or a 900-mg LD.