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Bronchiolar abnormalities are relatively common and occur in a variety of clinical contexts. There have been an increasing number of terms, some of which are redundant, used in referring to various forms of bronchiolar disorders. The purpose of this review is to provide an updated classification scheme to facilitate the clinical approach to patients with suspected bronchiolar disease.The spectrum of bronchiolar disorders is broader and more heterogeneous than previously recognized. In deciphering the potential significance of bronchiolar abnormalities, it is useful to distinguish those disorders in which the bronchiolar involvement is the dominant pathologic process, that is, primary bronchiolar disorders, from parenchymal or large airway diseases in which the bronchioles may become secondarily involved. Primary bronchiolar disorders include respiratory bronchiolitis, acute bronchiolitis, constrictive bronchiolitis, follicular bronchiolitis, diffuse panbronchiolitis, mineral dust airway disease, and a few other variants. Interstitial lung diseases with a prominent bronchiolar involvement include smoking-related interstitial lung diseases, cryptogenic organizing pneumonia, and hypersensitivity pneumonitis. Bronchiolitis can also be seen in large airway diseases such as chronic bronchitis, bronchiectasis, and asthma.In the clinical approach to a patient with bronchiolar disease, primary bronchiolar disorders should be distinguished from predominantly parenchymal or large airway processes with bronchiolar involvement. The number of patterns of bronchiolar response to injury is limited and these patterns are generally non-specific in regard to cause. Appropriate diagnosis and management of patients with bronchiolar disorders depend on judicious correlation of clinical, physiologic, and morphologic manifestations.