Sera from seventy-four adult patients with chronic or recurrent rhinosinusitis (mean duration 10.3 years) were tested for levels of total immunoglobulin and IgG subclasses. Fourteen (19%) had low levels of one of the major immunoglobulin classes and 23 (31%) had one or more IgG-subclass deficiencies, i.e. values less than the mean minus 2 standard deviations of a control population sample. Nineteen patients had low IgG3 levels. The group as a whole showed significantly low mean levels of IgG3, 46.9 +/- 19.5 mg/dl compared to 76 +/- 21 mg/dl for the controls, p < 0.0005 (Student's t-test). Since there was no clinical difference between those with and without IgG3 deficiency, there exists the possibility of an underlying immune defect, possibly involving heavy chain switching, in all these patients.