Abstract Collateral ventilation through alveolar vents, as a consequence of acquired and congenital diseases of sublobar bronchial obstruction, with obliteration, occurs relatively frequently. When collateral aeration persists over a period of years, it invariably results in emphysema and radiologic hyperlucence. This emphysema, in areas ventilated through collateral channels, is due to the pathophysiology of expiratory obstruction, in a check-valve manner, at the alveolar vents. Obliterative bronchitis, an acquired disease process, has been demonstrated by serial bronchial cross sections in a case representative of one form of bronchiectasis which is associated with hyperlucence of the peripheral pulmonary parenchyma. The association of bronchiectasis, obliterative bronchitis, collateral ventilation, and resultant emphysema and hyperlucence, is pointed out. Bronchial cyst formation, a developmental abnormality, is shown to induce collateral aeration of the parenchyma surrounding the cyst. The demonstration of increased pulmonary radiolucence, therefore, should always raise the possibility of collateral ventilation as a pathogenetic factor in the development of the emphysematous changes. In addition, the bronchial or bronchiolar obstructing mechanism which has isolated the bronchial lumen continuity from the surrounding parenchyma, and brought collateral ventilation into play, should be demonstrated whenever possible. As a corollary, when cases of bronchial cysts and of bronchiectasis are investigated, expiration films, with and without bronchography, should be used to demonstrate the air-trapping of emphysema of the surrounding parenchyma. In the early stages of emphysema, in which the microscopic criteria and evidence of alveolar changes are not clear-cut, roentgenologically demonstrated parenchymal hyperlucence, overexpansion and air-trapping substantiate the diagnosis of emphysema. A hypothesis relative to the pathology and pathogenesis of the “unilateral hyperlucent lung” is offered.