A new case of lymphocytic interstitial pneumonitis developed in the course of a persistent generalized lymphadenopathy syndrome is reported. The patient was a 30-year old Haitian woman with only her ethnic risk factor. Broncho-alveolar lavage showed high cellularity with mostly major lymphocytosis (76%) and a fall of the OK T4/OK T8 ratio to 0.23. The LAV was isolated from the lavage fluid lymphocytes on the same day and within the same culture time as from blood, using lymphocyte culture and measurement of reverse transcriptase activity in the supernatant fluid of cell cultures. This, together with the strongly positive (1/80) LAV serology in fluid as compared with blood (1/640), suggested that the LAV virus was directly or indirectly involved in the pneumonitis, being responsible for lymphocyte proliferation as it is in lymph nodes. No superinfection with a bacterial, fungal or other than LAV viral agent was found in blood or in lavage fluid. Lymphocytic interstitial pneumonitis is uncommon in AIDS or ARC (13 cases reported), but its incidence no doubt is underestimated, as it may be latent. It certainly accounts for the high lymphocyte count observed in broncho-alveolar lavage fluid in the absence of superinfection and, most probably, for many cases of so-called "non-specific pneumonia". In 1986, patients with apparently primary lymphocytic interstitial pneumonitis should be investigated for AIDS or ARC.