The pulmonary function records of 132 patients with cystic fibrosis followed for 5 to 7 years were reviewed. Changes in forced vital capacity, 1-sec forced expiratory volume, mean forced expiratory flow during the middle half of the forced vital capacity, and 1-sec forced expiratory volume as a percentage of forced vital capacity were examined. There was considerable variation in the rates of change, but the general pattern was consistent with a theory of exponential decline, mean forced expiratory flow during the middle half of the forced vital capacity showing the earliest and most dramatic changes. The pulmonary function of 33 patients (25 per cent) remained stable or improved throughout follow-up, possibly reflecting mild forms of lung disease or the efficacy of therapy. Twenty of these patients (15 per cent) maintained completely normal pulmonary function. The rate of decline in pulmonary function values, with progress of the disease, was steeper in the female patients.