We will be studying fondaparinux 2.5 mg subcutaneously every 48 hr in three distinct patient groups: 1) Acute kidney failure without hemodialysis, 2) Acute kidney failure (AKI) with intermittent hemodialysis (IHD) and 3) Acute renal failure with continuous renal replacement therapy (CRRT). All patients will be assessed for efficacy of the dose. Efficacy will be assessed by following clinically for any evidence of VTE, either deep venous thrombosis (DVT) or pulmonary embolism. In addition, lower extremity duplex studies will be performed at baseline and at the end of the study period to assess for DVT.