The mechanisms underlying the development of postoperative deep vein thrombosis remain to be fully elucidated. Previous studies have suggested that peroperative venous distension may be a factor associated with venous thromboembolism. In this study we have obtained high resolution ultrasound images of gastrocnemius and posterior tibial veins in 62 patients undergoing a range of general surgical procedures. From these we determined the changes in vein diameter occurring during the operative procedures, in response to induction of anaesthesia, and after completion of surgery. Veins showed no evidence of dilatation in response to the induction of anaesthesia, but by the end of the operative procedure showed distension of 22-28 per cent. Distension was most prominent in the gastrocnemius veins, and lesser distension was observed in the posterior tibial veins. In a series of patients who received an infusion of 1 litre of saline in addition to basal requirements, distension was 57 per cent compared with 22 per cent in the corresponding control group. Intraoperative venous distension is associated with factors that lead to deep vein thrombosis and may be involved in the mechanisms which result in the commencement of deep vein thrombosis.