To study the role of T cells in bronchiolitis obliterans organizing pneumonia (BOOP) and chronic eosinophilic pneumonia (CEP) and to examine the influence of differing racial background, T-cell subsets in bronchoalveolar lavage (BAL) fluid (BALF) and in peripheral blood of 8 Japanese patients with idiopathic BOOP and 5 with CEP were compared with those of 15 normal subjects. The BALF pattern in BOOP was characterized by a significantly high number and percentage of lymphocyte and by a low CD4 to CD8 ratio compared with patients with CEP and healthy volunteers. Patients with CEP showed a significantly higher percentage of BALF eosinophils compared with other groups. There was no significant difference in BALF CD4 to CD8 ratio between patients with CEP and volunteers. Two-color analysis of T-cell subsets revealed that CD3+HLA-DR+ cells (activated T cell) in BALF of patients with BOOP and CEP increased significantly compared with volunteers, while BALF CD3+CD25+ cells (interleukin 2 receptor+ T-cell) did not. In addition, BALF CD8+HLA-DR+ cells (activated suppressor/cytotoxic T cell) in patients with BOOP and CD4+HLA-DR+ cells (activated helper T cell) in patients with CEP were significantly higher than levels detected in healthy subjects. The percentage of CD8+CD57+ cells and the number of CD8+CD11b- cells (cytotoxic T cell) in BALF were significantly higher in patients with BOOP compared with patients with CEP and healthy volunteers. There were no significant differences in the expression of peripheral blood T-lymphocyte surface antigens among the groups. These findings indicate that cytotoxic T cells in Japanese patients with idiopathic BOOP and helper T cells in CEP appear in the lungs is consistent with a previous report in Caucasians, supporting the hypothesis that T cells may play an important role in the pathogenesis of these diseases.