This is a pilot study to evaluate safety of fondaparinux and enoxaparin in terms of bleeding and development of thrombocytopenia in the medical ICU population. We expect the null hypothesis to be proven true. One hundred subjects will be enrolled in this prospective, randomized, double-blind (concealed allocation) pilot study of consecutive medical ICU patients to one of two pharmacologic VTE prophylaxix treatment arms. (1) Primary endpoints include hemoglobin, hematocrit, & platelet counts. blood product utilization, bleeding complications (any), Steady State Functional Factor Xa activity (for post hoc analysis), non-study pharmacologic VTE prophylaxis (when estimated CLcr < 30ml/min), and days not treated with study drug or treated with alternative agent Secondary/ Additional Data to be collected include patient demographics, primary & secondary diagnoses, central venous access, and sequential compression device utilization.