Bronchiolitis includes a broad range of conditions with variable clinical, functional and morphological expression. Bronchiolar disease may be a primary or a secondary condition. Non-specific inflammatory changes are observed at the histology examination, involving respiratory and membranous bronchioles with or without extension to the alveoli. CT findings vary from subtle to quite marked abnormalities related to alveolar involvement, including small centrilobular lesions (nodular and branching linear opacities), ground-glass attenuation, and airspace consolidation. All are potentially reversible and treatable. Cicatrization processes may lead to constrictive bronchiolitis, an irreversible condition characterized by circumferential submucosal or periadventitial fibrosis causing an obstruction of the bronchiolar lumen. CT findings include decreased lung attenuation and expiratory air trapping within the areas of bronchiolar obstruction. The contrast with normally ventilated and perfused areas creates a mosaic perfusion pattern.