Bird fancier's lung (BFL) resulting from avian antigen exposure is a very common form of hypersensitivity pneumonitis. Its pathogenesis is modified by genetic polymorphisms located within the major histocompatibility complex, and also by smoking, which may decrease serum antibody response to inhaled antigen. Acute, subacute, and chronic presentations of BFL are recognized, but often overlap clinically. Continued antigen exposure in the chronic phase portends a worse prognosis. Chronic bronchitis symptoms may be part of the BFL clinical spectrum, and rhinitis may suggest an allergic component. The diagnosis of BFL is enhanced by a high index of suspicion of exposure to avian antigen, recurrent symptomatic episodes occurring 4-8 h after exposure, inspiratory "velcro" crackles on auscultation, weight loss, and positive IgG precipitins to the antigen. Characteristic findings on high-resolution computed tomography of the chest include centrilobular nodules, ground-glass opacification, and mosaicism due to air trapping. Bronchoalveolar lavage will classically show >25% lymphocytosis, a CD4/CD8 ratio of <1.0 and >1% mast cells in the acute phase. Lung biopsies, if obtained in the subacute phase of the disease, typically show loosely formed granulomas, giant cells, a lymphoplasmacytic interstitial infiltrate, and possibly some degree of fibrosis. In some patients, usual interstitial pneumonia or fibrotic non-specific interstitial pneumonia patterns may be seen on surgical biopsy. Skin testing, serological testing, and bronchial provocation tests for BFL frequently suffer from a lack of standardization. Effective treatment for BFL consists mainly of antigen avoidance, as corticosteroids likely do not alter long-term prognosis. Lung transplantation can be considered for progressive chronic disease refractory to medical measures.