Immunoglobulin G (IgG) subclass levels were measured in 58 patients with chronic obstructive pulmonary disease (COPD) and in 125 healthy controls. Total IgG values were significantly lower in the 27 COPD patients on steroid therapy compared with patients not taking steroids (8.31 (0.14) vs 9.80 (0.14), p less than 0.05), geometric mean (log SD). Total IgG (9.80 (0.14) vs 12.18 (0.16), p less than 0.005), IgG1 (5.87 (0.19) vs 6.68 (0.12), p less than 0.05) and IgG2 levels (2.75 (0.21) vs 3.70 (0.20), p less than 0.005) were significantly reduced in the COPD patients not taking steroids compared with the controls. IgG3 values were significantly elevated in smokers compared with nonsmokers in both the control and COPD groups. Fifteen COPD patients (25.9%) had a low level of one or more subclasses. IgG2 subclass deficiency was the most common, being present in 9 patients. A significant correlation was found between forced expiratory volume in one second (FEV1) and IgG2 subclass levels (r = 0.415; p less than 0.005). IgG subclass deficiencies may contribute to the development and progression of respiratory disease in COPD patients.