The majority of children with idiopathic thrombocytopenia (ITP) have an acute self-limiting course and no diagnostic test has been identified which will predict the course of thrombocytopenia and detect those with the chronic autoimmune form. The detection of autoantibodies directed against the platelet glycoprotein complex IIB/IIIa, may identify patients with chronic ITP. Serum anti-GP IIb/IIIa antibodies were assessed by the indirect MAIPA assay in 54 children with immune thrompocytopenia at initial presentation along with an additional 7 children previously diagnosed with chronic ITP, to determine if there was a difference in antibody positivity between acute and chronic ITP patients, and whether the identification of antibodies could be used as a predictive test at diagnosis. There was no significant difference in the percentage of antibodies detected in children classified with acute ITP (27/40-68%) compared to children with chronic ITP (13/21-62%, P > 0.05). Patients with acute ITP had significantly lower mean platelet counts at diagnosis compared to the chronic ITP group (16,225/mm3 vs 32,250/mm3, P < 0.05), though there was no significant difference in the bleeding manifestations between the acute and chronic ITP groups. Serum anti-GP IIb/IIIa antibodies are detected in a high percentage of children with ITP and autoantibodies appear to be involved in the pathogenesis of both acute and chronic ITP. The detection of anti-GP IIb/IIIa antibodies at diagnosis, however, does not appear to be a useful prognostic test in childhood ITP.