Acute bronchiolitis is the most common lower respiratory tract infection in young children and may be life-threatening in those with underlying cardiac or respiratory conditions. We evaluated the nasal and serum levels of human neutrophil elastase (HNE) in patients with acute respiratory syncytial virus (RSV) bronchiolitis and investigated the correlation of these levels with illness severity. Fifty-one patients (28 boys, 23 girls) with acute bronchiolitis positive for RSV by direct immunoenzyme assay in nasal secretions (Group A) were studied. Thirty healthy children (17 boys, 13 girls) constituted the control group (Group B). Subjects in both groups were matched for age and gender. The ages (mean+/-SE) in Groups A and B were 4.5+/-0.41 and 5.0+/-0.65 mo, respectively. Venous blood and nasal secretions were taken from patients in group A on 1, 5, and 15 days after admission and once from controls (Group B) for determinations of HNE in nasal lavage and serum, as well as white blood counts (WBC). The peripheral blood eosinophil and neutrophil counts were elevated in 22/51 patients (43.1%) and 15/51 patients (29.4%), respectively. In nasal lavage specimens, neutrophils represented>or=75% and eosinophils>2% of all cells in 42/51 (82.0%) patients and 11/51 (21.5%) patients, respectively. There was strong correlation between the level of HNE and the percentage of neutrophils in nasal lavage (r=0.92). The mean nasal HNE concentrations of the patients on 1, 5, and 15 days after admission were higher than those of Group B (p<0.0001, p<0.001, p<0.001, respectively). Mean serum HNE concentrations on 1, 5, and 15 days after admission were higher in Group A than in Group B (p<0.0001, p<0.0001, p<0.0001, respectively). Nasal and serum HNE concentrations showed no correlations with the clinical score of disease severity (r=0.28 and r=0.29, respectively). This study shows that (a) serum and nasal HNE concentrations were significantly higher in RSV bronchiolitis patients than in controls, (b) they did not return to normal after the respiratory symptoms had improved, and (c) they showed no significant correlations with clinical score of severity. The results indicate that neutrophils contribute significantly to airway inflammation in these subjects and HNE levels in serum and nasal lavage may be useful markers of inflammation in acute RSV bronchiolitis.