Four adult patients with biopsy-proven bronchiolitis were identified and prospectively evaluated. Each patient presented with the rapid onset (weeks to months) of severe respiratory disease that was clinically distinct from asthma, chronic bronchitis, bronchiectasis, cystic fibrosis, and emphysema. Bronchiolitis patients were evaluated by pulmonary function testing and bronchoalveolar lavage (BAL) before and after two months of treatment with 1 mg/kg/day of prednisone. Initial BAL results of bronchiolitis patients were compared to those of cigarette smokers with chronic bronchitis (n = 4), asymptomatic cigarette smokers (n = 5), and normal nonsmoking volunteers (n = 5). Neutrophils comprised 53 +/- 13 percent of the cells recovered by BAL in bronchiolitis patients but only 3 +/- 2 percent of the cells in chronic bronchitis patients, 1.5 +/- 0.6 percent of the cells in asymptomatic smokers, and 0.3 +/- 0.3 percent of the cells in normal volunteers (p less than 0.01, all comparisons). Moreover, prednisone produced a striking decrease in lower respiratory tract neutrophils (53 +/- 13 percent to 8 +/- 3 percent, p less than 0.05) in all bronchiolitis patients while lung function either improved (two patients) or remained unchanged (two patients). These findings suggest a central role for the neutrophil in bronchiolitis and argue that BAL may be clinically useful in the diagnosis and management of these patients.