Lymphocytic interstitial pneumonitis, a disorder of unknown cause, has been described in association with infection by the retrovirus human T lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV). This report describes the isolation of HTLV-III/LAV in bronchoalveolar lavage fluid and the quantitation of antibodies directed against HTLV-III/LAV in bronchoalveolar lavage fluid and peripheral blood of two patients with lymphocytic interstitial pneumonitis associated with acquired immune deficiency syndrome-related complex. The ratio of the concentrations of HTLV-III/LAV-specific IgG to total IgG in the bronchoalveolar lavage fluid of both patients was higher than that of the peripheral blood. These findings are consistent with the hypothesis that lymphocytic interstitial pneumonitis is sometimes associated with HTLV-III/LAV infection of pulmonary tissue that evokes a specific humoral immune response locally in the lung.