Low molecular weight heparin fractions have been demonstrated as efficient as low doses of standard heparin in preventing post-operative deep-vein thrombosis. Several clinical studies are, now, carrying out in patients with acute deep-vein thrombosis in order to assess efficacy and safety of low molecular weight heparin fractions as compared to standard heparin. In a randomised study including patients with deep-vein thrombosis treated either with low molecular weight heparin or with subcutaneously standard heparin, we demonstrated an effectiveness, in terms of thrombus reduction, of low molecular weight heparin as well as standard heparin. However, low molecular weight heparin seemed to be safer than standard heparin.