We examined the relation between serum levels of IgG subclasses and lung function, as determined by spirometry, lung volumes, the single-breath nitrogen test, and static recoil pressures, in 29 patients with IgA deficiency and repeated upper or lower respiratory tract infections. Four of the patients had decreased levels of IgG2, and two had decreased levels of IgG3. Two or more lung-function values were abnormal in each of these six patients and also in three others with normal levels of IgG subclasses. Low levels of IgG2 and IgG3 were significantly related to abnormal lung function (P less than 0.01). The 20 patients with normal lung function all had IgG-subclass levels above the lower range. There may be a causal relation between low levels of IgG subclasses and deterioration in lung function, suggesting that patients with combined IgA and IgG-subclass deficiencies may benefit from immunoglobulin prophylaxis.