A French multicentre, open, randomized trial was conducted in lung cancer surgery in order to test the optimal dosage regimen: Fraxiparine 3075 IU AXa (fixed dosage) and Fraxiparine 4100 or 6150 IU AXa (dosage adjusted for body weight only), over a period of 8 days. 75 patients were allocated to each group. Efficacy (Doppler ultrasonography at D0 and D8, controlled by bilateral ascending phlebography when positive) and safety, i.e. perioperative blood loss and postoperative bleeding complications were the main assessment criteria. The efficacy of the two treatment regimens was confirmed = no deep vein thrombosis and/or pulmonary embolism. No significant difference of safety was observed between the two groups: nevertheless fewer patients developed major bleeding complications in the Fraxiparine fixed dosage group (2 patients) than in the Fraxiparine adjusted dosage group (6 patients). Blood loss was comparable in the 2 groups; a statistical difference (p = 0.09) was showed between D0 and D2 in favour of Fraxiparine fixed dosage group. The results of this trial indicate that Fraxiparine administered at fixed dosage represents an effective and safe prophylaxis against fatal thromboembolism in patients undergoing oncologic thoracic surgery.