OBJECTIVE: Prophylactic administration of low-molecular weight heparin (LMWH) has been shown to reduce the occurrence of venous thromboembolism (VTE) among acutely-ill medical patients in the clinical trial setting. The purpose of this study was to examine outcomes and costs of LMWH thromboprophylaxis among medical inpatients in real-world clinical practice. METHODS: Using a large, geographically-diverse, multi-hospital U.S. database, we identified persons aged ?40 years who were hospitalized for ?6 days with an acute medical condition (including circulatory disorders, respiratory disorders, infectious diseases, or neoplasms) during calendar year 2000. From these patients we identified those who received either thromboprophylaxis with the LMWH enoxaparin or no thromboprophylaxis. Surgical patients, patients with conditions requiring anticoagulant therapy, and those medically ineligible for anticoagulation were excluded. We compared the incidence of VTE (defined as deep-vein thrombosis [DVT] or pulmonary embolism [PE]) and death during the hospital stay, as well as length of inpatient stay and total costs, in the two cohorts. RESULTS: We identified 215 patients receiving enoxaparin prophylaxis and 3,595 receiving no prophylaxis. The risk of VTE over the course of the hospitalization was 58% lower with enoxaparin prophylaxis than with no prophylaxis (3.3% versus 7.9%; p<0.001); there was no difference in the risk of death (7.4% versus 7.3%; p=0.96). Length of stay in hospital and costs of inpatient stay were nominally higher in the group receiving enoxaparin prophylaxis versus no prophylaxis (10.4 days versus 10.1 days; US$10,497 versus US$9,904), although the magnitude of these differences was not significant (p=0.40 for length of stay; p=0.29 for costs). CONCLUSIONS: Prophylaxis with LMWH was found to be effective in reducing the risk of thromboembolism in medical inpatients in real-world clinical practice; however, no corresponding reductions in length of hospital stay and inpatient costs were observed.