Plasma beta-thromboglobulin (beta TG) and thromboxane B2 (TXB2) level, platelet aggregation (PA) in platelet rich plasma, platelet survival time using 111Indium radiolabelled platelets and platelet sensitivity to prostacyclin (PGI2) were measured in chronic obstructive arterial disease (COAD) patients. Severity of the disease was assessed by the ankle pressure index using Doppler auscultation. Platelet survival time was shorter, plasma beta TG and TXB2 and the rate and extent of PA induced by ADP or 1-epinephrine (but not collagen) were greater in the patients than in controls. Beta TG was inversly correlated with the pressure index and positively with TXB2 indicating increased platelet TXA2 synthesis. Platelet sensitivity to PGI2 was similar in the patients and controls. These results indicate increased platelet consumption and enhanced in vivo platelet activation and PA in COAD. The enhanced activation correlates with the severity of the disease and the activated platelets presumably synthesize increased amounts of TXA2. It is therefore concluded that platelets might be involved in the pathogenesis of COAD. The normal platelet sensitivity to PGI2 suggests that the administration of the compound may improve the abnormal platelet function in COAD patients and may attenuate the progression of the disease.