A prospective open study was carried out on 30 pediatric patients with most severe chest disease whose serum immunoglobulin levels were normal. The patients entered into the study had had two or more documented episodes of pneumonia, and/or six episodes of bronchitis with fever within a year, and/or severe asthma (steroid-dependent), and/or hospitalization for chest disease for more than 30 days within the year preceding the study. Eleven patients had sinopulmonary infections, 19 had asthma. Twenty patients had low levels of one or two IgG subclasses: 11 were deficient in IgG3, three in IgG4, three in IgG3 + IgG4, and three in IgG2 + IgG4. Patients with low IgG subclass levels were distributed throughout the different clinical entities. These children had significantly longer periods of hospitalization than the patients in whom all IgG subclasses were within the normal range. They suffered more often from sinopulmonary infections. Asthmatic children with low levels of an IgG subclass reported more days with wheezing and needed more steroids than the children without subclass deficiencies.