It has been suggested that thrombotic events occurring in patients with chronic obstructive pulmonary disease could be partially due to abnormalities in blood coagulation and fibrinolysis, as well as to changes in platelet function. In the present study in vitro platelet aggregation, in addition to clotting activity, has been studied in 20 patients with chronic obstructive pulmonary disease and different degrees of alteration of blood gas analysis. The results obtained suggest that the presence of a condition of platelet hypoaggregability characterizes patients with marked hypercapnia and/or hypoxemia, whereas the improvement of respiratory indexes is followed by a trend to normalization, or, more often, to an increase of platelet activity.