Unfractionated heparin (UFH) binds to several plasma, platelet, and endothelial proteins, producing a highly variable anticoagulant response. For this reason, frequent anticoagulation monitoring and dosage adjustment are necessary during UFH administration. In contrast, low-molecular-weight heparins (LMWHs) exhibit less protein binding and provide more predictable anticoagulation with reduced need for patient monitoring and dosage adjustment. Therefore, LMWHs are potentially useful for anticoagulation therapy on an outpatient basis. Several recent clinical trials have compared LMWHs (administered primarily on an outpatient basis) and UFH for the treatment of venous thromboembolism. These trials have indicated that LMWHs possess efficacy and safety characteristics similar to intravenous heparin but are easier to administer. LMWH preparations vary considerably in their methods of preparation and pharmacological properties, and the relative efficacies of these various LMWHs remain to be determined by direct comparisons in randomized clinical trials.