The concepts of the pathological mechanisms in childhood idiopathic thrombocytopenic purpura (ITP) have, to a great extent, been based on clinical experience and on data generated in adults. Studies performed in children have demonstrated that platelet antigen-specific autoantibodies are present in chronic ITP and, to a lesser extent, in acute ITP. It is, however, likely that the mechanisms initiating the production of autoantibodies are different in the two entities. In acute ITP, production of autoantibodies and immune complexes is probably linked to a transient antiviral immune response. Chronic ITP in children is an autoimmune process which eventually is reversible in many cases. The initiating factors, as in other autoimmune disorders, are yet to be elucidated.