Validation of rigid-tube bronchoscopy with small-volume (5-ml increments not to exceed 300 ml) bronchial washing as a therapeutic adjunct was performed on six patients with cystic fibrosis, using serial tests of pulmonary function as a yardstick for assessment of efficacy. Two patients did not undergo the procedure and served as control subjects. All patients were characterized as having varying severity of pulmonary involvement. Large central airways were severely obstructed, and older patients had more trapped gas in their lungs. Hypoxemia and large alveolar-arterial oxygen pressure differences [P(A-a)O2] were due to inhomogeneity of alveolar ventilation. Results indicated that up to ten days to two weeks, bronchoscopic bronchial washing may in some instances improve maximal expiratory flow-volume curves and specific airway conductance and decrease P(A-a)O2 towards normal. Distribution of alveolar gas P(A-a)O2 towards normal. Distribution of alveolar gas became more homogeneous. We conclude that bronchoscopic bronchial washing may be effective in the management of patients with cystic fibrosis, by augmentation of their inadequate cleansing function of the conducting airways.