Idiopathic thrombocytopenic purpura occurs at all ages, in acute and chronic forms. Children mainly have the acute form, which usually follows a recent viral illness, occurs equally in both sexes, and generally resolves within six months. Chronic idiopathic thrombocytopenic purpura occurs more often in adults, often has an insidious onset, and shows a three:one female preponderance. Both forms are now thought to be due to an antiplatelet antibody, usually of the IgG class (platelet-associated IgG), which coats autologous platelets and leads to their phagocytosis and destruction by the reticuloendothelial system. In most patients, the spleen is the major site of the production of this platelet antibody and the destruction of the platelets. Many methods have been developed to detect this antiplatelet factor in the serum and on the platelets of patients with idiopathic thrombocytopenic purpura. Recent methods are becoming highly sensitive and may soon be simple and fast enough for routine clinical use and should significantly aid the diagnosis and management of these patients. Platelet-associated IgG levels appear significantly higher in patients with idiopathic thrombocytopenic purpura than in normal subjects, and in patients with nonimmune thrombocytopenia. Higher levels are also seen in children than in adults, and in acute cases than in chronic ones. Platelet-associated IgG levels also vary inversely with platelet count and platelet life span, can predict the disease course and response to therapy, and may predict neonatal consequences of maternal idiopathic thrombocytopenic purpura. Evidence of other alterations in immune status, as well as alterations in platelet function and HLA associations, remains controversial. Classic treatment with corticosteroids and splenectomy remains highly successful in most cases. More recent therapies include the use of immunosuppressants and alkaloid-coated platelets, plasma-exchange transfusion, and high-dose immunoglobulin. Overall, fewer than 5 percent of patients have severe hemorrhage or refractory or fatal disease.