An 82-year-old man with a one-year history of spontaneous ecchymoses and posttraumatic bleeding was found on physical examination to have a pulsatile abdominal mass. Ultrasonography revealed a large abdominal aortic aneurysm with a freely moving 1.5--2-cm intraluminal thrombus. Laboratory data disclosed intravascular hemolysis, disseminated intravascular coagulation, and a prolonged bleeding time. Further investigation of platelet function demonstrated decreased glass bead retention (0-15%), and reduced or delayed aggregation responses to adenosine diphosphate, epinephrine, and collagen. Studies of platelet factor 3 availability, antiplatelet antibodies, and aggregation response to ristocetin were normal. Transfusion of ten units of normal platelets failed to shorten the patient's bleeding time, despite a marked rise in platelet count. Glass bead retention studies on normal and patient blood were not altered by mixture with patient and normal platelet-poor plasma, respectively. Platelet dysfunction in the presence of arterial aneurysm does not appear to have been reported previously.