Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disease characterized by antibody-induced platelet destruction. To better define the role of antigen-specific assays in adult chronic ITP, we prospectively measured platelet-associated autoantibody against either glycoprotein (GP) IIb/IIIa or GPIb/IX in 282 patients with chronic ITP and 289 patients with thrombocytopenia of other causes. We divided chronic ITP into four subgroups: presplenectomy, mild (platelet count >30 000 micro (L-1) requiring no therapy), presplenectomy, severe (platelet count <30 000 micro L(-1) requiring therapy but not splenectomy), postsplenectomy, remission (postsplenectomy partial or complete remission without further therapy) and postsplenectomy refractory (required therapy after splenectomy failure). Positive results: total ITP group, 55.4%; presplenectomy, mild, 31.1%; presplenectomy, severe, 42.6%; postsplenectomy, remission, 50.0%; and postsplenectomy, refractory, 87.8%. In addition, the degree of positivity increased with the severity of the patient's disease. The assay had a minimum specificity of 84.4% if clinical factors, consistent with immune thrombocytopenia, were not considered in patients with thrombocytopenia associated with other diseases. However, if clinical factors consistent with immune thrombocytopenia were considered and only patients with questionable immune thrombocytopenia and patients 'lost to follow-up' were included in the false-positive group the specificity was 93.1%. We conclude that the presence of immune thrombocytopenia is highly probable if the immunobead assay is positive and that antigen-specific assays are diagnostically useful in adult chronic ITP.