The initiating event in unstable angina (USA) and non-Q-wave myocardial infarction (NQMI) is the rupture of an atherosclerotic plaque resulting in local thrombosis. The current standard treatment is the administration of aspirin and heparin. The introduction of low-molecular-weight heparin (LMWH) offers a potential alternative therapy. Clinical mals have begun to examine the efficacy and safety of using LMWH in the management of acute coronary syndromes. Two pivotal studies have evaluated the effects of LMWH preparations on patients with USA or NQMI: The ESSENCE and the TIMI 11B trials. These studies suggest that LMWH plus aspirin is more effective and safer than unfractionated heparin in preventing myocardial infarction, recurrent angina, or death. Because of these differences, the American College of Cardiology and the American Heart Association have updated their guidelines for the treatment of USA and NQMI.