CONTEXT: The classic histopathology of hypersensitivity pneumonitis (HSP) is well known but variations do occur and at times the diagnosis can be difficult. This is particularly true in the chronic stage of the disease. OBJECTIVE: To review the wide variety of histopathologic changes that can be seen in HSP and to offer a practical approach to diagnosis, including the diagnosis of recently described variants of HSP such as the so-called hot tub lung. DATA SOURCES: This review draws from the author's own experience and a concurrent search of national and international literature. CONCLUSIONS: The diagnosis of HSP can be made with confidence only in light of clinical, serologic, and radiographic data. A particular challenge for pathologists is the recognition of the disease in its chronic stage. In this stage, the identification of poorly formed granulomas in association with (1) a pattern of homogeneous linear fibrosis and (2) irregular fibrosis in a partially peribronchiolar distribution would facilitate the diagnosis.
