Platelet volume distribution curves were obtained in 20 control subjects and in 21 patients with idiopathic/autoimmune thrombocytopenic purpura. A striking increase in microthrombocytes as well as megathrombocytes was noted in 86% of patients on one or more occasions, particularly in the prsence of severe thrombocytopenia. The entire spectrum of platelet volume distribution curves noted in patients could be reproduced experimentally in rabbits following intravenous injection of anti-platelet antibody. Differential centrifugation studies with control subjects revealed that microthrombocytes were light platelets and megathrombocytes were heavy platelets. Electron microscopy in patients with thrombocytopenia revealed that microthrombocytes were composed of intact small platelets as well as platelet fragments. It is concluded that severe peripheral destruction of platelets is associated with an increase in microthrombocytes as well as megathrombocytes.