To study the role of T cells in diffuse panbronchiolitis (DPB), we investigated T-cell subsets in bronchoalveolar lavage fluid (BALF) or 33 patients with DPB, nine patients with bronchiectasis, and 20 healthy volunteers. BALF from DPB patients contained a higher percentage of neutrophils than that from patients with bronchiectasis or healthy volunteers, whereas the percentage of lymphocytes was similar in the three groups. DPB patients, however, had a higher number of lymphocytes and a reduced CD4/CD8 ratio compared with the other subjects. A two-color analysis of T-cell subsets in peripheral blood and BALF revealed a significant increase in the percentage and number of CD8+HLA-DR+ cells and in the number of CD4+HLA-DR+ cells in BALF of DPB patients. The expression of the adhesion molecules CD 11a and CD18 on lung CD3+ cells was enhanced over that on blood CD3+ cells in DPB patients. However, there was no significant difference in the expression of these antigens in peripheral blood or BALF among the groups. There was no significant relationship between BALF interleukin (IL)-8 and lymphocyte accumulation in the lungs of the DPB patients, whereas a significant correlation between the percentage of neutrophils and IL-8 levels in BALF of DPB patients was observed. After treatment with macrolide antibiotics, a significant reduction in the number of lymphocytes and activated CD8+ cells and an elevation in the CD4/CD8 ratio in BALF of DPB patients was observed. Our findings suggest an activation of CD8+ cells in the airway lumen of DPB patients, supporting the hypothesis that lymphocytes are important cellular components of bronchial inflammation in DPB.