In the GDR are at present living more than 800 patients with cystic fibrosis, 113 of them are in the age of 18 years and over. The prognosis is in first place depending upon the state of the lung. Thus, a continuous observation of the lung function is mandatory. The alterations of the lung are starting in the small airways (inflammation with ulceration, stenoses, obliteration) with primary disturbances of the relation between ventilation and perfusion, and with obstructive disturbances of the ventilatory distribution. The periphery of the lung is considerably inflated (increase of RV/TLC, trapped air). With progressing disease, obstructions are also observed in the larger airways, and due to progressing loss of parenchyma, VC, FEV1, and TLC become reduced. In about one third of the patients bronchial hyperreactivity is developing. The efficacy of treatment with bronchodilating drugs is not uniform and must individually be checked. The physical performance is reduced in an extent which depends upon the severity of the disease.