Treatment of unstable coronary artery disease with a combination of aspirin and intravenous heparin is now widely established but carries a significant failure rate, the origins of which lie in the fundamental characteristics of heparin and aspirin. The use of low-molecular-weight heparins (LMWHs) as an alternative to unfractionated heparin carries theoretical and practical advantages. Clinical trial evidence is now available to show that the LMWH enoxaparin, in a dosage of 1.0 mg x kg(-1) subcutaneously every 12 h for 2-8 days, is significantly more effective than unfractionated heparin, and that the benefits persist even after treatment ends. Ongoing trials will provide more efficacy data over both the short and the longer term. Enoxaparin at a dosage of 1.0 mg x kg(-1) every 12 h is as effective as higher doses but is associated with a much lower rate of major haemorrhage.