Acutely ill general medical patients are at moderate-to-high risk of venous thromboembolism (VTE); approximately 10-30% may develop deep vein thrombosis or pulmonary embolism, the latter being a leading contributor to deaths in hospital. Medical conditions associated with a high risk of VTE include cardiac disease, cancer, respiratory disease, inflammatory bowel disease, and infectious disease. Predisposing risk factors for VTE in medical patients include history of VTE, history of malignancy, complicating infections, increasing age, thrombophilia, prolonged immobility, and obesity. Unfractionated heparin (UFH), low-molecular weight heparin (LMWH), and fondaparinux sodium have been shown to be effective agents in the prevention of VTE in medical patients. In this setting, UFH has a higher rate of bleeding complications than LMWH. There is no evidence supporting the use of aspirin, warfarin, or mechanical methods to prevent VTE in medical patients. We recommend either LMWH or fondaparinux sodium as well tolerated and effective thromboprophylactic agents in medical patients.