A series of 400 consecutive patients subjected to reconstructive arterial surgery were randomly allocated into four equal groups in order to study the effect of dipyridamole and acetylsalicylic acid (ASA) in the prevention of deep venous thrombosis and pulmonary embolism. There were 11 drop-outs. In the dipyridamole-ASA group there were no thrombotic complications while in the control group without antithrombotic therapy five cases of deep venous thrombosis and two cases of fatal pulmonary embolism were encountered (p < 0.05). In the groups receiving either dipyridamole or ASA alone two and four cases of deep venous thrombosis, respectively, were found. It is concluded that treatment with dipyridamole and ASA combined is useful in preventing postoperative thrombotic complication after peripheral vascular reconstructive surgery.