In the past 4 yr, 16 adult patients were identified who had accelerated onset of a severe respiratory disorder (usually obstructive in nature) that was clinically distinct from the more commonly encountered chronic obstructive disorders (e.g., chronic bronchitis, emphysema, asthma, bronchiectasis, cystic fibrosis, and alpha 1-antitrypsin deficiency). These patients, termed patients with "bronchiolitis," underwent pulmonary function testing, bronchoscopy with bronchoalveolar lavage (BAL), and open lung biopsy. Although lung biopsy findings varied somewhat among the patients, each biopsy contained a prominent component of bronchiolitis. Pulmonary function testing and BAL were also repeated after 3 months of treatment with oral prednisone (1 mg/kg/day). Initial BAL neutrophil percentages were significantly higher in the bronchiolitis group (54 +/- 10%) than in smokers with chronic bronchitis (3.9 +/- 1.0%) or in normal nonsmoking volunteers (0.8 +/- 0.5%) (p less than 0.01, both comparisons). Eleven of 15 patients with bronchiolitis had significant improvement (greater than or equal to 15% increase in FEV1) in their lung function after prednisone treatment. Furthermore, this "responder" subgroup had a significant reduction in BAL neutrophil percentages after treatment with prednisone (46 +/- 15% to 6 +/- 3%, p less than 0.05). Finally, the neutrophil products collagenase and myeloperoxidase were detected in the BAL fluid of patients with bronchiolitis. These findings suggest a central role for the neutrophil in the pathogenesis of bronchiolitis and emphasize the utility of BAL in the identification of these patients.