Low-molecular-weight-heparin fractions are prepared from standard unfractionated heparin and are thus similar to unfractionated heparin in many aspects. The main advantages of this new class of antithrombotic agents as compared with unfractionated heparin are: (1) an improved bioavailability and a prolonged half-life, which alleviate cumbersome laboratory monitoring and may permit one single daily subcutaneous injection; (2) an improved efficacy-to-safety ratio, with less bleeding despite similar or improved efficacy. While low-molecular-weight heparin should replace unfractionated heparin for preventing postoperative thromboembolism, some unresolved issues remain to be addressed in specific trials before low-molecular-weight heparin can generally replace unfractionated heparin for all indications. These issues include the use of low-molecular-weight heparin in patients with arterial thrombosis, unstable angina, or myocardial infarction (usually in conjunction with thrombolytic treatment), and in patients with symptomatic pulmonary embolism, as well as formal cost-effectiveness analyses substantiating the advantages of the new agents. The potential of using low-molecular-weight-heparin outpatient treatment of established deep-vein thrombosis should be scrutinized from an economic and logistic point of view because two large-scale controlled trials have suggested both efficacy and safety.