Seventy children with cystic fibrosis were studied over a 5-year period to assess the relationship between serum immunoglobulin G levels and progression of cystic fibrosis lung disease. Patients were grouped according to their serum IgG values (low, normal, or high) and evaluated with serial pulmonary function testing, radiographic and immunologic studies, and clinical observation. The children with persistent hypogammaglobulinemia G showed significantly better lung function, better weight for age, fewer hospitalizations for pulmonary exacerbations, less colonization with Pseudomonas aeruginosa, and slower decline in pulmonary functions than did age-matched patients with normal or high IgG levels. Death occurred in five of eight (63%) patients with hypergammaglobulinemia, three of 30 (10%) with normogammaglobulinemia, and one of 32 (3%) with hypogammaglobulinemia. No deaths occurred in the 15 patients with persistent hypogammaglobulinemia. These data indicate that children with cystic fibrosis and hypogammaglobulinemia G have milder lung disease and slower deterioration in pulmonary function than do age-matched patients with normal or elevated immunoglobulin G values. The mechanisms accounting for this finding are unclear.