 Fondaparinux is a parenteral anticoagulant drug approved for the prophylaxis of venous thromboembolism in high risk medical patients. A relevant proportion of these patients have renal insufficiency andor thrombocytopenia which represent independent risk factors for bleeding. The risk of bleeding may be increased when fondaparinux is administered to patients with a reduced renal function andor low platelet count. A lower dose of fondaparinux, 1.5 mg daily, has been approved for patients with renal insufficiency defined by a creatinine clearance between 20 and 50 mLmin. However, to our knowledge, there are no clinical studies that have specifically evaluated prophylaxis with fondaparinux in acutely-ill medical patients with a moderate to severe thrombocytopenia. The scope of this study is to evaluate the safety of fondaparinux in high risk hospitalized medical patients with a moderate to severe thrombocytopeniada defined by a platelet count between 100,000uL and 30,000uL. 