Childhood ITP is an acquired hemorrhagic disorder with a heterogeneous clinical course. We measured PAIgG levels in 20 children with ITP (7 acute, 13 chronic). Both groups had significantly greater PAIgG values than age-matched normal subjects and thrombocytopenic controls (P less than 0.001). In addition, PAIgG values in chronic ITP were significantly lower than those in acute ITP (P less than 0.003). Serial PAIgG values were obtained in some patients; most returned to normal in association with clinical recovery. The measurement of PAIgG is useful in the diagnosis and follow-up of childhood ITP. PAIgG values may assist in differentiating acute and chronic disease in children.