Hypersensitivity pneumonitis (HP) is caused by inhalation of environmental antigens. Farmers and bird keepers are most frequently affected by this desease. The HP diagnosis is based on clinical symptoms (cough, dyspnea) in a person exposed to environmental antigens, and the presence of characteristic changes in high resolution chest computed tomography (HRCT) (bilateral, mosaic, ground glass opacities in the middle and lower lung zones, ill-defined centrilobular nodules and the sign of air-trapping on expiration). This type of HRCT pattern is most frequently found in the patients with subacute HP. Bronchioloalveolar lavage fluid (BALF) examination is helpful in establishing the HP diagnosis, when the increased total number of cells, with the predominance of T lymphocytes (> 50%), and the increased number of neutrophils (> 3%) and mastocytes (> 1%) are found. The presence of specific serum precipitins increases the likelihood of HP. In case of atypical clinical presentation, lung biopsy is recommended. The diagnostic criterion of HP is the presence of ill-defined non-necrotising granulomas, after excluding other granulomatous lung diseases. The prevention and treatment of HP is based on the elimination of the antigen from the environment. Corticosteroids may contribute to the improvement in the acute and sub-acute form of the disease but their long term effectiveness is uncertain. The prognosis of HP patients is generally perceived as good, especially in those patients in whom antigen avoidance is possible. Nevertheless, in some patients progressive pulmonary fibrosis and development of severe respiratory insufficiency is observed. Med Pr 2016;67(4):517–527