A patient with pseudothrombocytopenia due to the anticoagulant EDTA is described. The thrombocytopenia was secondary to platelet agglutination and required the presence of EDTA in a critical concentration and the patient's plasma. Both homologous and autologous platelets were affected by this plasma factor in the presence of EDTA. The platelet agglutination was temperature independent and could be prevented by divalent cationsa. The agglutination phenomenon was irreversible. The platelet count in oxalate, not heparin, and citrate anticoagulated blood was normal. The patient's clinical course was benign and no hemorrhagic complications occurred. We believe this is the first reported case of pseudo- thrombocytopenia associated with a vasculitis. The diagnosis of this disorder, its implications and the pertinent literature is discussed.