Thromboembolic complications are the most frequent associated pathologies after knee replacement. The secondary deep vein thrombosis in the knee arthroplasty is often low symptomatic or asymptomatic and, sometime, it could lead to fatal pulmonary embolism. This is the main purpose recommending an antithrombotic prophylaxis. In this study 214 patients operated for knee arthroplasty and receiving low molecular heparin therapy were enrolled. They were clinically and echo/radiologically monitored for thromboembolic and/or hemorrhagic complications on the 2nd, 7th, 14th and 45th day from surgery. Results showed that only 45% of the cases developed DVT (deep vein thrombosis) out of which 17% could be objectively verified by ultrasonography and phlebography. In only one case massive pulmonary embolism with fatal evolution was discovered. It was not found any major bleeding except in four cases in which local haematomas were discovered (most probably related to anticoagulant treatment). This study concluded that the administration of low molecular weight heparin after knee arthroplasty significantly reduces the risks of thromboembolism in conditions where the increase of hemorrhagic complications doesn't result statistically significant.