Venous thromboembolism is a frequent, life-threatening, postoperative complication of hip-fracture and total-knee-replacement surgery. Fondaparinux is a synthetic polysaccharide that selectively binds to antithrombin, the primary endogenous regulator of blood coagulation. Low molecular weight heparins, such as enoxaparin, are less specific inhibitors of coagulation. In patients undergoing hip-fracture surgery, fondaparinux is more effective than once-daily enoxaparin as prophylaxis for venous thromboembolism. Fondaparinux (25 mg/day s.c.) was also more effective than enoxaparin (30 mg s.c. b.i.d.) as prophylaxis for venous thromboembolism in elective knee surgery. These differences may be explained by the fact that there is less prophylaxis cover with enoxaparin, as it has a much shorter duration of action than fondaparinux. Thus, with the present dosing regimens, fondaparinux is probably preferable to enoxaparin for the prevention of venous thromboembolism.