Bronchus-associated Lymphoid tissue (BALT) has been reported to be present in the lungs of patients with rheumatoid arthritis (RA). However, little is known about the structure and cellular distribution of BALT in this disease, so we investigated these points using immunohistochemical methods. The subjects were eight RA patients with BALT in biopsy specimens and a histologic diagnosis of follicular bronchiolitis. Seven patients had cough and purulent sputum, and four patients had positive sputum cultures. BALT was histologically composed of four distinct regions, which were the lymphoepithelium, the dome area, the follicular area, and the parafollicular area. Surface IgM+ B cells were predominant in the follicular area, whereas IgA+ cells were scattered in the dome and parafollicular areas. T cells were mainly found in the parafollicular area (CD4+ > CD8+), and most of them expressed the T Cell receptor alpha beta (alpha beta TCR). These findings were similar to those described previously for BALT in diffuse panbronchiolitis, which manifests as a chronic respiratory infection. The present study indicated that extrinsic stimulation as well as alterations of the immune response are involved in the development of BALT in RA, although the exact mechanism requires further clarification.