Data on 16 sibling pairs with cystic fibrosis were analyzed to test the hypothesis that early treatment of this condition improves prognosis. Younger siblings' conditions were diagnosed before 1 year of age, usually before the onset of pulmonary disease. Older siblings' conditions were diagnosed after 1 year of age and after the onset of pulmonary disease. Although the sibling pairs received similar treatment, comparison at 7 years of age showed that the younger siblings had significantly better chest roentgenogram scores, total clinical scores, residual lung volumes, and ratios of residual volume to total lung volume. Younger siblings also required fewer hospital admissions to control their lung disease. The results suggest that, in general, early initiation of therapy is beneficial for patients with cystic fibrosis.