Lowering of forced expired volume in one second as a percentage of vital capacity (FEV1%VC) and check-valve phenomenon of expiratory flow curve (CVPh) are clinical signs of bronchial collapse. In this study the correlations between these functional parameters and morphologic findings are studied. The study comprises autopsy cases in which lung function had been tested 1.5 years on average before death. In 138 cases emphysema was graded on papermounted macrosections and in 52 cases bronchi were studied by morphometry. 19 of the latter were analysed more thoroughly; 9 had shown CVPh. The results were as follows: 1. Collapse phenomena occur more frequently in cases with destructive emphysema than in cases without. However, it is not exclusive to the former. 2. FEV1 and CVPh correlate with hypertrophy of bronchial glands. 3. Cases with CVPh show hypertrophy, not atrophy, of the walls of central bronchi. It follows that bronchial collapse phenomena are mainly influenced by increased bronchial secretion of viscous mucus and not by emphysematous destruction of lung parenchyma. On forced expiration, mucus plugging probably leads to a decrease of intrabronchial pressure in the downstream bronchi. Thus, the downstream bronchi may collapse because the equal pressure point between intrabronchial and intrathoracic pressure shifts from the central to more peripheral bronchi.