In this review, mainly those studies are discussed which have been prospective and randomized. Emphasis will be given to studies which have used objective diagnostic endpoints to evaluate effectiveness. From these 207 studies, it appears that in elective general surgery low dose heparin is superior to Dextran and to electrical calf muscle stimulation, while one small study suggests that electrical calf muscle stimulation is superior to Dextran. In addition, low dose heparin appears to prevent 125I fibrinogen leg scan detected thrombus formation more successfully than oral anticoagulants begun soon after surgery. There has been only one study comparing low dose heparin prophylaxis with the effect of other drugs in elective hip surgery. In this study, heparin prophylaxis was discontinued after 20 patients, because there was a high incidence of thrombosis and bleeding.