IgG subclass concentrations were measured in 25 extrinsic asthmatic children because IgG subclass deficiency, which reflects delayed maturation of the immune system, might have a role in pathogenesis, and other forms of immunodeficiency, such as IgA deficiency, have been associated with atopic disease. Serum total IgG, IgM, and IgA concentrations were measured by nephelometry and IgG1, IgG2, IgG3, IgG4 concentrations of the patients' sera were measured by enzyme-linked imnunosorbent assay (ELISA). IgG1, IgG2, and IgG3 were found to be below normal ranges in 8%, 16%, and 12% of asthmatic patients, respectively 28% of cases had one or more values more than three standard deviation points below the mean for age-matched controls. IgG4 concentrations were found to be raised in only 8% of patients. Partial IgA deficieny was found in one of the patients. Recurrent respiratory tract infections were observed in two of the seven patients with IgG and IgG subclass deficiencies. It was concluded that IgG subclass deficiency might cause an impairment in the clearance of adsorbed allergnes and have a role in the pathogenesis of the disease.