BACKGROUND AND OBJECTIVES: Immunoglobulin G (IgG) subclass deficiency has been reported in patients with bronchial asthma and is associated with recurrent respiratory tract infections. Aspirin-sensitive asthma (ASA) which affects 10% of adult asthmatics, asthma runs a chronic course with frequent asthma exacerbations, often related to respiratory infections. We performed this study to identify the prevalence of IgG subclass deficiency and evaluate the association between recurrent asthma exacerbations and IgG subclass deficiency in ASA. SUBJECTS AND METHODS: We measured serum levels of IgG and IgG subclass in 26 aspirin- sensitive asthmatic patients (15 steroid used and 11 steroid not used) and 55 healthy controls using nephelometry. Reference values of each IgG subclass was defined as cumulative percentile between 2.5% to 97.5% of controls. RESULTS: Total IgG, IgG1, IgG2 and IgG3 of aspirin-sensitvie asthmatics, were significantly lower than for those of controls (p<0.05, respectively). However, there were no significant differences in total IgG and IgG subclass concentrations, between steroid-not-used asthma group and controls (p>0.05, respectively). CONCLUSION: Lowered levels of IgG, IgG1, IgG2 and IgG3 were noted in ASA sensitive asthma patients, which might be associated with use of steroid. Further studies will be needed to evaluate their clinical significance.