1. It has been suggested that CYFRA21-1, a cytokeratin subunit 19 fragment, is potentially useful for diagnosis and monitoring of lung carcinoma. However, serum levels of CYFRA21-1 are also increased in a high proportion of patients with interstitial lung disease. In this study we measured CYFRA21-1 levels in bronchoalveolar lavage fluid from 10 normal subjects, 18 patients with idiopathic pulmonary fibrosis and 14 patients with sarcoidosis, and determined whether any relationship exists between CYFRA21-1 levels in bronchoalveolar lavage fluid and clinical parameters. 2. CYFRA21-1 levels in bronchoalveolar lavage fluid were significantly higher in patients with sarcoidosis (mean value 8.3 ng/ml, P < 0.01) and idiopathic pulmonary fibrosis (42.5 ng/ml, P < 0.005) than in normal controls (1.0 ng/ml). Moreover, higher CYFRA21-1 levels in bronchoalveolar lavage fluid were found in sarcoidosis patients in radiological stage 2 or 3 than in those in stage 1. In patients with idiopathic pulmonary fibrosis, there was a significant correlation between CYFRA21-1 levels, and percentage of inflammatory cells in bronchoalveolar lavage fluid (r = 0.56, P < 0.05) and the magnitude of the alveolar--arterial oxygen pressure difference [P(A-a)O2] gradient (r = 0.66, P < 0.01). 3. Serial bronchoalveolar lavage samples were obtained from six patients with clinically active pneumonitis after they had undergone systemic corticosteroid therapy. CYFRA21-1 levels were significantly lower after these patients exhibited clinical improvement (P < 0.05). 4. These findings suggest that the level of CYFRA21-1 in bronchoalveolar lavage fluid is a useful marker for the clinical diagnosis of pneumonitis, and is also adequate for the evaluation of disease activity, especially over the course of treatment.