Hypersensitivity pneumonitis or extrinsic allergic alveolitis is an immunologically mediated lung disease caused by repeated inhalations of organic antigens. The basic histological lesion consists of a diffuse mononuclear cell infiltration of alveolar wall, alveoli, terminal bronchioles, and neighboring interstitium. The inflammation is often followed by granulomas, which then may progress to fibrosis. Unlike other infectious and noninfectious granulomatous disorders, hypersensitivity pneumonitis is limited to the lung. The disease occurs more frequently in men than in women and children. The acute form of hypersensitivity pneumonitis, characterized by fever, chills, myalgias, cough, and dyspnea, may be confused with acute pneumonitis. Although there is no single radiological, physiological, or immunologic test specific for hypersensitivity pneumonitis, the diagnosis can often be suspected on the basis of a compatible temporal relationship between pulmonary symptoms and a history of environmental or occupational exposure. Once the diagnosis is suspected, the presence of serum precipitating antibodies (immunoglobulin [lg] G), suppressor cytotoxic lymphocytosis in bronchoalveolar lavage (BAL) fluid, and granulomatous alveolitis in lung biopsy specimens is extremely helpful in confirming the diagnosis. For patients in whom the diagnosis is confirmed, avoidance of the causative antigen is the best therapy, although corticosteroids are used to suppress inflammation. Once the fibrosis has developed, the patient may gradually develop respiratory failure or cor pulmonale, possibly resulting in death.