In order to clarify the pulmonary involvement in rheumatoid arthritis (RA), bronchoalveolar lavage (BAL) was performed in 52 RA patients, in conjunction with pulmonary function test (PFT) and high-resolution CT (HRCT) of the chest. The results were as follows: 1. Out of nine RA patients with no evidence of lung involvement on HRCT, six (66.7%) showed abnormal differentials of the cells obtained through BAL. 2. RA patients with advanced interstitial lung disease on HRCT had significantly higher BAL fluid (BALF) cellular concentrations, increased percentage of BALF neutrophils and decreased BALF CD 4/CD 8 ratio, as compared with those with no or mild lung involvement. 3. In the non-smoking RA patients who also demonstrated no obstructive patterns on PFT, positive correlations were observed between %VC and the BALF CD4/CD 8 ratio (R = 0.481) as well as the BALF CD4+Leu8- (helper T) cell ratio (R = 0.497). On the other hand, a negative correlation was detected between %VC and the BALF CD8+CD11b- (cytotoxic T) cell ratio (R = -0.533). 4. In those with the increased percentage of BALF eosinophils and/or with increased BALF CD4/CD8 ratio, V25/Ht was depressed significantly, as compared with other patients. Likewise, those with severely depressed value of V25/Ht tended to have significantly elevated BALF CD4/CD8 ratio, as compared with other patients with normal V25/Ht value. Thus, through these detailed studies of BAL cellular differentials and BALF lymphocyte subsets, the diversity of pulmonary involvement was well demonstrated in RA.