Total IgA, IgM, IgG, and IgG subclasses were measured in 136 consecutive adults with recurrent respiratory infections. Sinus and chest radiographs were also obtained. IgG antibodies to H. influenzae and tetanus toxoid were determined in 27 immunodeficient subjects. Fifty-eight of the 136 were deficient in some immunoglobulin isotype, three in IgA, three in IgM, and 52 in total IgG or one or more IgG subclasses. The most common IgG deficiencies were single IgG subclass deficiencies, particularly IgG3 or IgG4, and a mild decrease in total IgG (between 450 and 650 mg/dL). In 21/27 subjects with IgG deficiency, the response to booster immunization was blunted, even when IgG1 and IgG2 were normal. Thirty-eight patients were smokers, 37 being > 40 years of age at the onset of respiratory infections. Twenty patients were nonsmokers, 19 being < 40 years of age at the onset. In conclusion, adults with recurrent respiratory infections frequently have some variant of IgG deficiency, often associated with a functional impairment of specific antibody response. Smoking may represent a risk factor for IgG deficiency in adults.