Bronchiolo-alveolar carcinoma (BAC) development was observed in 4 (9.3 %) of the 43 patients with idiopathic fibrosing alveolitis (IFA). The observed process was characterised by the following pattern: the absence of bronchorrhea, rapid progression accompanied by paraneoplastic reactions. BAC occuring in patients with IFA is morphologically characterized by its development in the stage of honeycomb lung. There is an hypothesis of sclerotic changes in BAR playing a role in IFA , since in the stage of honeycomb lung the cells of terminal airways, bronchioles and alveoli may be a subject to transformation. Open lung biopsy with morphological examination in rapidly progressing IFA, especially in the case of paraneoplastic reactions, is recommended.