To investigate the efficacy of i.v. IgG treatment in pediatric patients with inflammatory lung disease, a prospective, controlled clinical trial was carried out over a 2-year study period. Patients were enrolled on the basis of severe clinical symptomatology. After 1 year of conventional treatment, the patients received 400 mg/kg per month of an i.v. IgG product containing only trace amounts of IgG3 in addition to their regular treatment throughout the second year. Significant clinical improvement, as documented by duration of hospital stay (first year 27.8 days, second year 4.9 days), use of antibiotics (132.8 versus 30.9 days) and use of steroids (21.4 versus 0.7 days) could be observed. Data obtained on a subgroup of patients with lgG3 deficiency were analysed separately. These results indicate that patients with severe chest disease who have lgG3 deficiency will also benefit from i.v. IgG treatment. The mode of action cannot be attributed to replacement of the respective isotypes, but is probably due to the effect of i.v. IgG in preventing repeated viral infections.