Lung transplantation is an accepted therapeutic modality in end-stage lung disease. Presently, histologic examination of tissue by transbronchial biopsy remains as the definitive diagnostic procedure for determining rejection. To begin addressing the usefulness of flow cytometric analysis of bronchoalveolar lavage fluid (BALF) in acute lung rejection, we prospectively studied the expression of markers on lymphocytes from BALF samples removed from 10 lung and heart-lung transplant recipients and compared their pattern with that of BALF lymphs obtained from normal volunteers (Norm) and nonrejecting heart transplant recipients (HT) who were receiving similar immunosuppressive regimens. Compared with both Norm and HT subjects, CD4+ lymphocytes in the BALF of lung transplant recipients was significantly reduced. A greater percentage of the CD4+ lymphocytes in nonrejecting lung transplant subjects also expressed the interleukin-2 receptor, but only during the early post-transplant period, suggesting possible reactivity to persistent donor cells. However, the CD8+ lymphocytes were increased only in lung transplant recipients undergoing acute lung rejection. We conclude that the immunologic milieu is indeed altered in the transplanted lung. Further studies in lung transplant recipients are required to evaluate the role of flow cytometry in the early detection of acute lung rejection.