The lungs of 9 patients with cystic fibrosis were studied by morphometric techniques to determine the amount of bronchiectasis, emphysema, pneumonia, bronchial gland enlargement, and small airways narrowing and density. The severity of these processes, which should account for the clinical picture of air-flow limitation in these patients, varied greatly among patients of different ages. Bronchiectasis was present in all children, but the amount of bronchiectasis did not appear to increase with age. Mild destructive emphysema was seen only in adults, but many younger patients had overinflation without destructive emphysema, as recognized by increased mean linear intercept (interalveolar distance). Despite clinical evidence of mucous hypersecretion, bronchial gland enlargement was present only in some patients. The younger patient showed normal small airways, but children and younger teenagers tended to have dilated small airways. Older teenagers and adult predominantly had stenosis of small airways. Varying patterns of pulmonary pathology are seen in cystic fibrosis and may be related to rapidity of progression of disease.