BACKGROUND AND OBJECTIVE: IgG subclass deficiency has been reported in patients with bronchial asthma and is associated with recurrent respiratory tract infections. This study was done to identify prevalence of IgG subclass deficiency and to evaluate the possible difference between atopic and non-atopic asthmatics. Subjects and METHODS: We measured serum levels of IgG and IgG subclass in 35 asthmatic patients and 50 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 and IgG2 of asthmatics were significantly lower than for those of controls(p<0.05, respectively). In atopic asthmatics, compared with non-atopic asthmatics, IgG4 level was significantly higher (p<0.05). The frequency of IgG subclass levels below the reference value was eight (22.9%) of 35 asthmatics. CONCLUSION: IgG, IgG1 and IgG2 were significantly lower in asthmatic patients. Some patients had IgG subclass levels below reference value. Further studies will be needed to evaluate their clinical significance.