The aim of this study was to report serum immunoglobulin (Ig) and IgG subclass levels in a large pediatric population with cystic fibrosis, and relate these to measures of disease severity. Total immunoglobulin levels were measured in 154 patients, and IgG subclass levels were measured in 136 patients and compared to age-related normal population data and to levels reported in previously published studies of children with cystic fibrosis. Clinical data were also collected: genotype; height, weight, and BMI standard deviation scores; FEV(1) (as percent predicted); Shwachmann-Kulczycki (S-K) and Northern chest X-ray scores; and Pseudomonas aeruginosa infection status. The clinical well-being of patients with hypo- or hyper-gammaglobulinemia was compared with age- and sex-matched control patients who had normal levels of gammaglobulin. IgG subclass levels were measured, and the results were compared with previous studies. Eleven patients had hypergammaglobulinemia (7.8% compared with 0-69% in the published literature). Patients with hypergammaglobulinemia had lower FEV(1) percent-predicted values, and worse S-K and Northern chest X-ray scores than controls. Three patients had hypogammaglobulinemia (1.9% compared with 0-10.8% in the published literature). There was no difference in any clinical parameter between controls and those with hypogammaglobulinemia. Nineteen patients (14%) had low levels of IgG1, and 40 patients (29%) had low levels of IgG2. The low percentage of patients with abnormally high immunoglobulin levels probably reflects the improved respiratory status of today's children with CF. The low percentage of those with low IgG probably reflects better nutritional status. The finding of worse lung function and clinical scores in patients with hypergammaglobulinemia agrees with the published literature. The high percentage of patients with low IgG2 was unexpected and was not previously reported. The clinical significance of this in patients with CF is unknown.