Patients undergoing major abdominal surgery, particularly for malignancy, are at increased risk of venous thromboembolism. Haemostatic markers of coagulation are raised for several weeks after surgery. A higher dose of low-molecular-weight heparin than normally used for thromboprophylaxis is effective in preventing post-surgical VTE in patients with cancer with no compromise on bleeding. Four weeks' of thromboprophylaxis with the LMWH dalteparin is significantly more effective than standard (1 week) thromboprophylaxis in preventing proximal DVT. A meta-analysis of studies comparing 4 weeks' with 1 week of thromboprophylaxis showed that prolonged thromboprophylaxis with LMWH following major abdominal surgery for malignancy significantly reduces the risk of late occurring DVT.