Using venous occlusion impedance plethysmography, leg blood flow was measured in seventeen men undergoing transurethral prostatectomy under either amethocaine spinal anaesthesia (SAB) or nitrous oxide-halothane general anaesthesia (GA). Mean leg blood flow doubled (206% of pre-operative control value) following induction of SAB, remained elevated throughout surgery (146% of control at the end of operation) and had returned to pre-operative levels (94%) by two hours postoperatively. Under GA, mean blood flow rose gradually during surgery to 136% of control by the end. However, mean blood flow was only 28% of control at one hour postoperatively, rising to 66% of control by two hours. Analysis of variance showed a significant difference in blood flow changes between the two groups (Greenhouse Geisser P = 0.005). The early postoperative fall in leg blood flow in the GA group might be important in the aetiology of postoperative deep vein thrombosis.