ABSTRACT. The concentrations of IgG subclass immunoglobulins were determined by radial immunodiffusion in serum from 126 patients with cystic fibrosis (CF). The results were compared to values from age-matched healthy children and adults and correlated to patients age, duration of chronic Pseudomonas aeruginosa infection and lung function parameters. Fifty-two percent of the patients had an elevated concentration of at least one of the IgG subclasses; IgG1 28%, IgG2 16%, IgG3 18% and IgG4 48%. There was significant correlation between elevated serum levels of IgG2, and to a lesser extent IgG3, with decreased lung function (for FEV1; p=0.0001, and p=0.001 respectively) and high levels of antipseudomonas precipitins (p=0.008, and p=0.002). A similar correlation was not found for IgG1 and IgG4. IgG subclasses vary in their ability to promote phagocytosis and to activate complement and it is possible that individual differences in the IgG subclass pattern could explain the variable course of this disease.