The object of this study was to evaluate the effectiveness of the immuno-suppressor Azathioprine (AZ) on chronic and severe pulmonary sarcoidosis, with a persistent activity resistant to prolonged corticotherapy. The study was done on 10 patients (4 women, 6 men) afflicted by an histologically proven and chronic sarcoidosis, resistant to steroid treatment. The treatment consisted of a daily oral intake of 150 mg of AZ for six months. Its effectiveness was evaluated before and after treatment, in comparison with a control group and a steroid-treated group of sarcoid patients. Serologic and alveolar functional and immuno-biologic tests were performed in 8 cases according to the activity criteria defined at the IXth International Congress on sarcoidosis. No clinical or hematological side effects were observed; a clear and prolonged radiological and clinical amelioration was observed in 7 out of 10 cases and in 3 cases a restoration of sensibility to the tuberculin skin test; in the 8 cases a significant improvement (p less than 0.01) was noted after the sixth month of treatment only in the alveolar fluid in the following parameters: ACE, all the proteins studied and the percentage of lymphocytes.