One hundred and seventy-six patients scheduled for elective major abdominal surgery were randomized to two prophylactic regimens to prevent postoperative thromboembolism. All patients were screened with the 125I-labelled fibrinogen uptake test, and thromboembolism was verified by ascending phlebography and/or perfusion/ventilation lung scintigraphy. In the group of patients receiving low-dose heparin treatment (5000 units twice daily subcutaneously) 12 per cent developed thromboembolic complications. In the other group, where low-dose heparin treatment was supplemented with graded compression stockings only 2 per cent developed thromboembolism. It is concluded that the combination of low-dose heparin and the use of graded compression stockings is superior to heparin alone in preventing thromboembolism following major abdominal surgery.