Pulmonary cysts 3 to 25 mm in diameter are found in average in 13 % of patients with sub-acute hypersensitivity pneumonits (HP). Lymphocytic infiltration by means of air valve effect causes bronchiolar obstruction, hich leads to the formation of the cysts. The cysts are also found in chronic HP. The rupture of a cyst often causes a pneumothorax. A case of chronic HP is presented. The first signs of the disease were noted in the childhood at the age of 10. But the final accurate diagnosis was not established until the age of 40. The patient repeatedly referred to pulmonology and tuberculosis medical institutions, where she was diagnosed with idiopathic fibrosing alveolitis, idiopathic interstitial pneumonia, suspected tuberculosis, mainly based on the description of radiographs and tomograms. Later, all these diagnoses were cancelled. The diagnosis was verified only by morphological examination of a fragment of the lung tissue obtained during surgery for pneumothorax on the left. At that time the disease has acquired its chronic course. A retrospective analysis of the course of the disease and the reasons for delayed diagnosis was carried out. Key words: hypersensitivity pneumonitis, clinical course, diagnostics.