An open study was carried out in 14 patients with peripheral arterial disease to investigate the effects of prolonged therapy with picotamide on platelet activity. Patients received daily oral doses of 900 mg picotamide for 1 month, 600 mg per day during the second month and 300 mg per day from the third to the sixth month of the study. Measurements were made before and during therapy of blood coagulation parameters and factors influencing platelet function, i.e. plasma beta-thromboglobulin and serum thromboxane B2. The results showed that there were no significant variations in platelet count, prothrombin time, partially activated thromboplastin time, presence and amount of fibrinogen in blood, and antithrombin III. Examination for fibrinogen degradation products was constantly negative and unaltered during therapy. Although plasma beta-thromboglobulin values did not vary significantly, there was a significant and progressive reduction throughout treatment in serum levels of thromboxane B2.