To investigate the lung function during positive expiratory pressure (PEP) physiotherapy in cystic fibrosis, the resistance tube of the PEP-mask was inserted into the expiratory outlet of our lung function equipment. This enabled us to measure a variety of lung function variables, while the lung function equipment functioned as a PEP-mask. We studied 12 patients and found that during PEP-mask physiotherapy functional residual capacity (FRC) increased significantly (p less than 0.02). There was a decrease of washout volume (WOV) (p less than 0.05), lung clearance index (WOV/FRC) (p less than 0.001) and volume of trapped gas (p less than 0.05), whereas total lung capacity, vital capacity, tidal volume and residual volume did not change significantly. It is concluded that in cystic fibrosis PEP-mask physiotherapy evens the intrapulmonary distribution of the ventilation and opens up regions, that are otherwise closed off. The results support the clinical observation that PEP-mask physiotherapy increases the transcutaneous tension of oxygen and the expectoration of sputum.