The association of various respiratory disorders with disturbances in immunoglobulin G (IgG) subclass levels is increasingly being recognised. This was a prospective study of the IgG subclass levels in 71 patients (37 white, 34 black) with various respiratory disorders associated with obstructive airways disease. Ten white patients with adult cystic fibrosis were studied, 4 of whom were colonised with Pseudomonas aeruginosa. Alterations in individual subclass levels were seen in these patients and abnormalities noted included a decrease in IgG3 and/or an increase in IgG1 and/or IgG2 levels. Of the 17 black and 16 white patients with asthma, 2 had absent IgG4 levels associated with severe and recurrent respiratory infections, 2 had deficient IgG3 and 1 decreased IgG1 levels. Several patients had increased levels of several IgG subclasses, of which IgG1 was the most commonly affected. Both atopy and recurrent chest infections occurred most often in the latter group of patients. In the studies of 17 black and 11 white patients with bronchiectasis, all but 3 white patients were shown to have some alteration in IgG subclass levels. The commonest deficiencies were an absence of IgG4 (3 cases), and an absence of all subclasses (2 cases). One of the latter patients had an associated deficiency of total IgG and IgM, and the other demonstrated pan-hypogammaglobulinaemia. Abnormalities of IgG subclass levels appear to be commonly associated with several respiratory disorders including recurrent infections, atopy and bronchiectasis.