We evaluated the efficacy and safety of the administration of low-dose unfractionated heparin(LDUH)for the prevention of pulmonary thromboembolism after lung cancer surgery. We operated on 206 patients with primary lung cancer for 8 years;128 males and 78 females, mean age:69.9±8.8 years. All patients were administrated LDUH 5,000 units every 12 hours from the operation day until the day when the patient could walk around the floor. No patients suffered from clinical pulmonary thromboembolism in this period. The duration of treatment was 4.6±2.6 days and the chest tube duration was 5.4±3.0 days. We experienced post-operative intra-thoracic bleeding in 2 patients during the previous 4 years. Based on this experience, we introduced new eligibility criteria;we discontinued LDUH administration on the operation day if diffuse adhesion in the thoracic cavity was observed at operation or intraoperative blood loss was over 500 ml. The dose of LDUH was decreased to 2,500 unit every 12 hours if the postoperative bleeding was over 400 ml on the operation day or the patient's body weight was less than 40 kg. After introduction of the new criteria, no severe bleeding complications occurred during the latter 4 years.