To determine the effect of vascular surgery on platelet function, a perioperative investigation of 37 patients with peripheral arterial disease (PAD) was performed using the Stagnation Point Adhesio-Aggregometer (SPAA). The SPAA provides well defined flow conditions. By means of dark field microscopy platelet microthrombus formation can be directly observed and measured continuously. Mathematical evaluation of resulting growth curves renders the constants for adhesion and aggregation, Kpw and Kpp, respectively. The PAD patients were divided into 2 groups: diabetics (n = 9) and nondiabetics (n = 28), and were examined perioperatively at regular intervals (average: n = 8). Preoperatively all patients received aspirin and low molecular weight heparin (LMWH). As of surgery and up to the third postoperative day all patients received unfractionated heparin (UH), at which time LMWH was resumed. Plasma fibrinogen concentration was also determined. Data obtained preoperatively were compared to those of 40 healthy volunteers (without medications). In the present study a significant increase (p < 0.001) in platelet reactivity was verified in PAD patients in spite of aspirin and LMWH administration. As of the first and up to the 8th day after surgery, a marked increase in platelet adhesivity and aggregability as well as plasma fibrinogen concentration and a concomitant decrease in platelet count was observed. Maximum values were obtained during intravenous administration of UH. Thrombocytopenia (< 150,000/ml) was observed in 12 patients. The hypercoagulability response to vascular surgery observed in the present study occurred in spite of therapy with aspirin and heparin. Our findings indicate the need for further improvement in conventional therapy and the SPAA as a useful tool in monitoring the effectiveness of current as well as of future inhibitors of platelet function.