We have studied the effect of extradural analgesia on postoperative venous thrombosis in patients undergoing knee arthroplasty. Forty-eight patients were allocated randomly to receive either general anaesthesia or extradural analgesia with local anaesthetics for 3 days. All patients wore compressive elastic stockings and no anticoagulant drugs were administered. Bilateral venography was performed 10 days after surgery. Continuous extradural analgesia did not impede mobilization of the patients. One case of non-fatal pulmonary embolism occurred in a patient who received general anaesthesia. The use of continuous extradural analgesia resulted in a significant difference in the total incidence of deep vein thrombosis (18% compared with 59% after general anaesthesia (P = 0.02)). The incidence of calf vein thrombosis was 12% compared with 45% after general anaesthesia (P = 0.05).