We analysed BALF cell findings in 9 patients (group A: 5 patients without lung disease after bone marrow transplantation (BMT), group B: 4 patients with lung disease after BMT) before and after BMT. Before BMT, BALF cell findings in group A were almost normal, whereas a relative increase of lymphocytes was seen in group B. Although total cell counts and cell composition in group A changed little after BMT, CD4/CD8 ratios in BALF lymphocytes decreased. In contrast, a relative increase of lymphocytes and neutrophils was seen in group B and there was variation of CD4/CD8 ratios in BALF lymphocytes after BMT. We there studied the BALF lymphocytes of 3 patients in group A and 4 patients in group B after BMT by means of 2-color analysis. Among CD4+ cell populations, CDw 29+ cells were decreased in both groups after BMT. A relative increase of BALF-CD4+ HLA-DR+ cells and CD8+ HLA-DR+ cells was seen in group A, but was not seen in group B. These findings suggest that there is abnormality of local immunity in the lung after BMT.