The cause and pathogenesis of thrombotic thrombocytopenic purpura (TTP) are still not clearly defined. It has been believed that the inciting cause is primary endothelial cell injury, intravas-cular platelet agglutination, or a concurrence of both (1). In this study, we measured the circulating endothelial cells (EC) in venous blood from one patient with TTP during acute illness and after he achieved sustained remission. EC were isolated and counted using S-Endo l, and EC specific monoclonal antibody coupled to micro-magnetic beads as previously describ ed (2). A Zl-year-old woman was admitted with headaches, confusion and spontaneous bruising. A blood count revealed that the haemoglobin was 66 gll, the reticulocytes were 8"/", the platelet count was 9 x 10efi. A peripheral-blood smear demonstrated marked fragmentation of red cells. Renal function was normal, latic dehydrogenase was 1,239 mU/ml. Coagulation studies revealed no evidence of disseminated intravascular coagulation. Tests for human immunodeficiency virus and antinuclear anti-body were negative.