We have determined IgG subclass concentrations in 100 patients with aspirin-induced asthma and 80 healthy controls. Patients on chronic corticotherapy (n = 64) had significantly lower total IgG levels than patients not receiving steroids (n = 36) or controls. Corticotherapy was not associated with changes in the subclass distributions. In patients, the most striking finding was elevation of IgG4. It was not related to corticotherapy or serum IgE levels. The rise in IgG4 was accompanied by a modest, though statistically significant, depression of IgG1. No changes of IgG2 and IgG3 concentrations were observed. Thus, aspirin-induced asthma is characterized by a distinct pattern of distributions of IgG subclasses. It is suggested that in aspirin-induced asthma elevation of IgG4 might result from chronic antigenic stimulation, of viral origin, and that determination of IgG subclass distribution might be of clinical interest.