The aim of the study was to evaluate the effect of two antithrombotic therapies on platelet function and on coagulation in patients with nonvalvular atrial fibrillation (NVAF). Twenty patients with NVAF were treated with aspirin (300 mg/day) and clopidogrel (75 mg/day) for 2 weeks immediately followed by oral anticoagulation (target international normalized ratio 2.0–3.0). Parameters of platelet function and coagulation were evaluated before antithrombotic therapy, at the end of aspirin plus clopidogrel and during subsequent anticoagulation treatment. Aspirin plus clopidogrel significantly inhibited platelet aggregation, fibrinogen receptor activation and release of P-selectin and prolonged in vitro bleeding time (p < 0.01). Coagulation parameters (platelet-dependent thrombin generation, antithrombin III, thrombin-antithrombin III complex, prothrombin fragment 1 + 2) were not significantly affected. During the subsequent oral anticoagulation phase platelet function was not substantially reduced; however, coagulation parameters were significantly inhibited (p < 0.001). The results indicate that combined antiplatelet therapy is superior to aspirin monotherapy in inhibiting platelet function but does not seem to substantially modulate coagulation cascade in patients with NVAF.