Interest in low molecular weight heparins as potential antithrombotic agents has been stimulated by two observations. These were that low molecular weight heparins have a different anticoagulant profile from unfractionated heparin and that some low molecular weight heparins are less haemorrhagic in animal models than unfractionated heparins for equivalent antithrombotic effects. Subsequently, it was shown that low molecular weight heparins inhibit platelet function and impair vascular permeability less than unfractionated heparin and that low molecular weight heparins have a longer biological half-life than unfractionated heparin. A number of low molecular weight heparins have been evaluated in clinical trials in general surgery, orthopaedic surgery and in the treatment of venous thrombosis. Low molecular weight heparins are highly effective in orthopaedic surgery where they appear to be more effective than unfractionated heparin. Low molecular weight heparins have also been shown to be either as effective or more effective than unfractionated heparin in preventing post-operative thrombosis following general surgery. In preliminary studies, low molecular weight heparins appear to be as effective as unfractionated heparin in the treatment of venous thrombosis but larger studies are required using clinically relevant outcome measures.