The effect of lumbar epidural analgesia and of general anaesthesia on the peroperative velocity of flow in the femoral vein was measured with Doppler ultrasound technique in 38 patients undergoing retropubic prostatectomy. The patients were randomly allocated to the epidural or the general procedure. The 125I-fibrinogen test was used to detect deep vein thrombosis (DVT). Epidural analgesia was associated with a significant (ca. 120%) increase in the femoral venous flow velocity. The peak flow showed a moderate (47%) increase, whereas the minimum flow was greatly increased (188%). The increase was significant already 2 min after induction of the analgesia and the rate continued to rise for about 11 min. General anaesthesia significantly reduced the flow velocity in the femoral vein. Immediately after induction of the anaesthesia the velocity approached zero, but gradually rose, and after about 8 min was stabilized at level 40% below the preoperative velocity. The peak flow velocity in all patients of this group fell by 24 to 40%. Only 2 of 17 patients with epidural analgesia, but 11 of 21 with general anaesthesia had postoperative DVT. In the 11 patients with DVT the mean minimum velocity of flow was decreased (by 85%), but in the 10 without DVT it was increased (by 175%). The difference was not statistically significant. Increased velocity of flow in the femoral vein, especially of minimum flow, seems to counteract development of DVT.