Acute bronchiolitis is the main cause of emergency visits and hospitalizations in infants. Recent data suggest that neutrophil- and eosinophil-mediated inflammations were part of bronchiolitis pathophysiology. Apart from the defined risk factors, few was known on the underlying pathophysiology, which might point out the differences observed in the severity of the disease. The aim of this study was to assess whether the clinical severity of acute epidemic bronchiolitis in young infants might be related to a specific underlying inflammatory process. Total and differential cell counts, IL-8, eotaxin, eosinophil cationic protein (ECP) and albumin levels were assessed at the time of admission in bronchial secretions from 37 infants (median age 17 wk) with acute bronchiolitis. Outcome severity variables were: hypoxemia, Silverman score, tachypnea, feeding alteration, and duration of hospitalization. Neutrophils predominated, and eosinophils were present in 54% of the infants. IL-8 levels strongly correlated with ECP and albumin levels. Albumin levels were correlated with ECP and eotaxin levels. IL-8 levels were higher in infants with hypoxemia and inversely related with SaO(2) levels. IL-8 and albumin levels significantly rose with respiratory rate, and Silverman score. IL-8, albumin and ECP levels were significantly higher in infants hospitalized >/=7 days. Furthermore, IL-8 levels were correlated with the duration of hospitalization. Neither cell counts nor eotaxin levels were related to the severity criteria studied. This study suggests that IL-8-associated airway inflammation significantly contributed to the severity of acute epidemic bronchiolitis.