Lymphocyte activation may be involved in interstitial inflammatory processes in the lungs. We analyzed lymphocytes in bronchoalveolar lavage fluid obtained from 5 patients with idiopathic bronchiolitis obliterans organizing pneumonia and from 7 control subjects. Lymphocytes were analyzed by two-flow cytometry. Differential cell counts showed that the percentage and the number of lymphocytes was higher in the patients (32.9 +/- 21.9%, 12.8 +/- 8.32 x 10(4) cells/ml) than in the controls (8.31 +/- 3.66%, 0.69 +/- 0.39 x 10(4) cells/ml, p < 0.05). In particular, the patients had many more CD8+ S6F1+ lymphocytes (which are considered to be activated cytotoxic T lymphocytes; 32.2 +/- 11.8%, 42.0 +/- 26.5 x 10(3) cells/ml) than did the controls (6.31 +/- 1.69%, 0.42 +/- 0.25 x 10(3) cells/ml, p < 0.01). These data indicate that activated cytotoxic T lymphocytes may be important in the pathogenesis of bronchiolitis obliterans organizing preumonia.