Bronchoalveolar lavage (BAL) cell populations were determined in 47 immunosuppressed patients during episodes of pulmonary disease. Thirty six patients had AIDS and 11 had conventional causes of immunosuppression. Pulmonary disease was due to a variety of infectious and noninfectious causes and was similar in both groups. In the AIDS patients, the mean BAL cell proportions were 64.2 ± 3.7 percent alveolar macrophages (MACS), 28.7 ± 3.4 percent lymphocytes, 3.5 ± 1.8 percent polymorphonuclear cells (PMN) and 1.6 ± 0.6 percent eosinophils (EOS). The non-AIDS group had similar findings in the BAL, with 59.3 ± 8.3 percent MACS, 34.8 ± 7.2 percent lymphocytes, 5.5 ± 1.7 percent PMN and 0.4 ± 0.2 percent EOS. The most striking finding in each group was a significant increase in both the proportion and absolute number of lymphocytes compared to controls. This was in marked contrast to the peripheral blood findings of lymphopenia. There was no characteristic cell profile diagnostic of any specific pulmonary disease. There was also no direct relationship of the cells present to respiratory symptoms, roentgenographic abnormalities or survival from pulmonary disease. This study demonstrates that although there was wide individual variation in lavage findings, a local pulmonary inflammatory reaction consisting predominantly of lymphocytes occurs in the immunosuppressed host during episodes of lung disease. The significance of this lymphocyte alveolitis and the complex host pathogen interaction responsible for determining the cell populations present in the lungs of these patients requires further study.