The serotonin release test using platelets from a panel of donors and autologous platelets after recovery was used to detect platelet antibodies in 42 children with idiopathic thrombocytopenic purpura. The test was done with sera obtained in the thrombocytopenic phase, in partial recovery, and after recovery. The test results for sera from 23 (55%) of 42 patients with thrombocytopenia were positive against platelets from the donors; 14 (61%) of 23 were positive against autologous platelets; and 28 (88%) of 32 were positive against either donor or autologous platelets. Complete recovery occurred in 17 (52%) of 33 patients whose sera had positive tests against the donor platelets at the time of diagnosis. Postrecovery sera from all patients had negative tests against donor platelets, and two (12%) had positive tests against autologous platelets. These results indicate that childhood idiopathic thrombocytopenic purpura is an autoimmune disease caused by platelet antibody that usually disappears with recovery from the thrombocytopenia.