In fibrosing pneumonitis, respiratory insufficiency is due to both fibrosis and inflammation induced pulmonary fibrosis. There is no treatment that can suppress fibrosis, so the current treatment of fibrosing pneumonitis--corticosteroid and/or immunosuppressive drugs--aims at reducing pulmonary inflammation and thus at slowing down the development of fibrosis which cannot regress. Therefore, it is necessary to determine the respective parts of inflammation, potentially reversible, and of fibrosis which is not. Most of the time, respiratory insufficiency cannot be prevented and requires long-term oxygen therapy. In a few patients, lung transplantation must be discussed.