A survey of clinical data shows that substitution with intravenous immunoglobulins constitutes an essential further step in the therapy of patients with immunodeficiencies: patients with primary and some secondary immunodeficiencies clearly benefit from intravenous immunoglobulin substitution. Some other diseases including idiopathic thrombocytopenic purpura and the Kawasaki syndrome also constitute indications for this kind of treatment. In other cases (infections in newborn preterm infants, children with AIDS, chronic inflammatory demyelinizing polyneuropathies and the Guillain-Barré syndrome and children with refractory seizures) there are reasons to believe that intravenous immunoglobulin therapy can be of benefit, although bigger controlled studies are still necessary to allow definitive conclusions to be reached. Further attempts will have to be made to improve understanding of the mode of action of intravenously administered immunoglobulin.