Ten children and young adults with humoral immunodeficiency disorders were enrolled in a blind crossover study of the effect of intravenous methylprednisolone in preventing immediate adverse infusion reactions to intravenous immunoglobulin. The patients selected had all experienced frequent adverse reactions previously. Each patient received two infusions of intravenous immunoglobulin at least 4 weeks apart. Infusions were of constant volume and rate for each patient. Intravenous methylprednisolone in a dose of 1 mg/kg bodyweight was given 20 min prior to one of the infusions only. A significant decrease in the severity of infusion-associated immediate adverse reactions was seen following the administration of methylprednisolone (P less than 0.01). Interruption of the infusion was necessary for only one patient when methylprednisolone prophylaxis was given, but temporary cessation of the infusion was required for eight of the 10 patients when methylprednisolone was not given (P less than 0.05).