In AIDS a variety of severe pulmonary disorders may occur. The authors report 110 cases of bronchoalveolar lavage (BAL) in 43 AIDS and 41 ARC. In AIDS P. carinii pneumonia is the major cause of respiratory illness. BAL alone is a safe and valuable tool for diagnosis of P. carinii pneumonia and others opportunistic infections. Moreover, pulmonary hemorrhage diagnosed by the finding of hemosiderin laden macrophages, is very suggestive of broncho-pulmonary Kaposi' sarcoma. Finally, BAL demonstrates a severe depletion of T4 lymphocytes and an increased number of T8 lymphocytes. The T8 lymphocytosis is observed whatever the pulmonary involvement (nonspecific alveolitis, opportunistic infections, Kaposi's sarcoma), and is also found in ARC, and lymphocytosis, open lung biopsy shows a lymphoid interstitial infiltration with respect of the alveolar septa, thus differing from the classical lymphoid interstitial pneumonia described by Carrington. The prognosis of lymphocytosis in ARC remains unknown.