There is experimental evidence that low molecular weight heparin fractions are as effective as standard unfractionated heparin but cause less bleeding. In order to determine the efficacy and safety of low molecular weight heparins, we have conducted an overview of published randomized trials in which a low molecular weight heparin was compared to either placebo or to unfractionated heparin. In the two trials comparing low molecular weight heparin prophylaxis with placebo, the incidence of leg vein thrombosis was substantially reduced with low molecular weight heparin and there was no increased bleeding risk. There have been five trials which compared low molecular weight heparin with unfractionated heparin in high risk surgical patients. In one of these studies there was a statistically significant reduction in postoperative venous thrombosis in patients who received the low molecular weight heparin. In the remaining studies, thrombosis rates were relatively low in all groups and no difference was detected between low molecular weight heparin and unfractionated heparin. We found a significant relationship between ex vivo anti-factor Xa levels and clinically important bleeding, r = 0.54. We conclude that low molecular weight heparin is an effective method of prophylaxis and has a negligible risk of bleeding provided that the anti-factor Xa levels do not exceed 0.2 units per ml. There have been only three published randomized trials on the efficacy of low molecular weight heparin in the treatment of venous thrombosis. Much larger studies are required to establish the efficacy and safety of low molecular weight heparin fractions in the treatment of established venous thromboembolism.