In 30 unselected male patients, bronchoscopic aspect and lung function parameters (notch in FEV1, FEV1/FIV1%, discrepancy between airways resistance and FEV1, club-shaped resistance curves) were correlated to determine the validity of lung function in the diagnosis of a central bronchial collapse. There were no strong correlations; only the club-shaped resistance curve and FEV1/FIV1% less than 60 were a little more often seen in the presence of central bronchial collapse. It is concluded from these results, and with regard to the known results from bronchial pressure measurements, that the functional parameters mentioned above are good indicators of a flaccid tracheobronchial system but not typical for a central bronchial collapse alone. Additional methods (bronchoscopy, bronchial pressure measurements) are needed to locate the major pressure drop and to select the adequate therapy (conservative in the case of pure peripheral or most mixed situations, chirurgical in special cases with pure central bronchial collapse). The reactions of a collapsible tracheobronchial system to bronchodilating drugs are described and their help in location of the major pressure drop is discussed.