Inhibition of activated coagulation factor X (FXa) is an attractive target for antithrombotic treatment strategies, because of the central position of FXa in the coagulation cascade. Most of the now available anticoagulant drugs have inhibitory effects not only on FXa, but also on thrombin. With the development of pentasaccharides, a new class of antithrombotic agents has emerged that acts by specific inhibition of FXa and lacks activity against FIIa. Fondaparinux, the first synthetic short-acting pentasaccharide, has been evaluated, in a large phase II and III clinical programme concerning prophylaxis and treatment of venous thromboembolism and also in phase II studies in patients with acute coronary syndromes. Idraparinux, the long-acting pentasaccharide, has been studied in a dose-finding study in patients with established deep-vein thrombosis and phase III studies are now planned in patients with venous thromboembolism and in patients with atrial fibrillation.