In this prospective study 364 patients were subjected to different types of arterial reconstructive surgery. The patients were randomly divided into four groups according to the type of antithrombotic medication given. During the hospitalization period there were no reocclusions in the dipyridamole/ASA group of 93 patients compared with 12 reocclusions in the control group of 86 patients. The difference was statistically highly significant. The difference between the control group and the given dipyridamole or ASA was not statistically significant. Dipyridamole and ASA were tolerated well by the patients in this clinical trial. This was the first clinical investigation to prove the effectiveness of dipyridamole and ASA medication in the prevention of reocclusions in arterial reconstructive bypass surgery.