The effect of extracorporeal circulation on platelet count and size (mean platelet volume) was studied in 65 patients (nine bleeders and 56 nonbleeders). In addition to the above, in 20 of the patients platelet aggregation response to adenosine diphosphate, collagen, and ristocetin was measured. Platelet counts dropped postoperatively both in the bleeder and in the nonbleeder groups. The difference between them was not significant. However, the bleeders had a significantly lower mean platelet volume (7.7 ± 0.84 versus 8.68 ± 1.1 fl) and lower volume percentage of platelets in whole blood (0.075% ± 0.02% versus 0.116% ± 0.04%) (p < 0.05) than the nonbleeders. None of the bleeders had a volume percentage of platelets in whole blood higher than 0.095%. All 20 patients studied for platelet functions had an abnormal postoperative aggregation response to adenosine diphosphate, collagen, and ristocetin. Three patterns of disturbed response to ristocetin were observed: grade I, delayed onset (14 patients); grade II, incomplete aggregation (five patients); and grade III, total lack of aggregation (one patient). All patients with delayed-onset response to ristocetin had a normal bleeding time, whereas the six patients with grade II and III responses had prolonged bleeding times and three of them had clinically significant bleeding. Factor VIII procoagulant activity, factor VIII-related antigen, factor VIII-ristocetin cofactor, and factor VIII two-dimensional electrophoresis were found normal, which suggests that the von Willebrand-like reaction to ristocetin observed in this study was caused by a defect in platelet membrane rather than by factor VIII changes.