A 23-year-old woman with the antiphospholipid syndrome developed severe thrombocytopenia (14 x 10(9)/l) and microangiopathic hemolytic anemia after plasma exchange. A relation was suspected between rising anticardiolipin level and development of thrombotic microangiopathy. The mechanism responsible may be interference between anticardiolipin antibody, platelet membrane and endothelial cell.