The prevention of post-operative venous thromboembolic accidents is well documented. This is in contrast to prophylaxis in medical patients. In surgical patients, prophylaxis should take into consideration the risk associated with the operation and the patient's risk factors. Low molecular weight heparins (LMWH) are frequently used since they more convenient and at least as efficiacious or somewhat superior to subcutaneous administration of unfractionated heparin (UH). Gradued compression stockings are recommended for low or moderate risk of thrombosis. Their use in combination with LMWH is limited to high risk patients. Oral anticoagulants are mainly used when long term prophylaxis starting with LMWH administration is required. Several meta-analysis and consensus conferences in France, Europe or North-America have provided recommendations, the comparison of which shows a great homogeneity. It is generally accepted that the recommendations made for surgical patients may be extrapolated to medical patients. The medical situations associated with an increased risk of thrombosis which are well documented are cancer, myocardial infarction, cardiac insufficiency, ischemic cerebro-vascular accidents with paralysis of lower limb(s), acute respiratory insufficiency in patients admitted to intensive care unit, prolonged immobilisation in elderly invalid patients and paralysis of the limbs due to spinal trauma. Prophylactic treatment in medical patients is still empirical. The use of LMWH in these patients is related to results obtained when unfractionated heparin is used and in rare controlled clinical trials where LMWH were compared to UH.