Although airway inflammation is recognized as a key feature of asthma, the characteristics of airway inflammation in children with acute severe asthma are not well defined. The aim of this study was to describe the characteristics of airway inflammation in children with an acute exacerbation of asthma using sputum cell counts and fluid-phase measurements and to examine the changes in these parameters upon resolution of the exacerbation. Children (n = 38) presenting to the Emergency Department with acute asthma underwent successful sputum induction using ultrasonically nebulized normal saline (n = 22), or expectorated sputum spontaneously (n = 16). Sputum induction was repeated at least 2 wk later when the children had recovered (n = 28). Sputum portions were selected, dispersed and total and differential cell counts performed. Neutrophil elastase and EG2-positive eosinophils were assessed and fluid-phase eosinophil cationic protein (ECP), myeloperoxidase (MPO), interleukin-8 (IL-8), and IL-5 were measured. During the acute exacerbation the median (range) total cell count was 8.4 x 10(6)/ml (0.5 to 190.3), and fell significantly at resolution to 1.3 x 10(6)/ml (p < 0.01). The inflammatory cell infiltrate was mixed and included eosinophils (0.8 x 10(6)/ml), neutrophils (3.3 x 10(6)/ml), and mast cells. EG2(+) cells were high and correlated with the degree of airflow obstruction (r = -0.5, p = 0.02). They decreased significantly at resolution as did supernatant ECP (1,078 versus 272 ng/ml), suggesting that eosinophils were activated during the exacerbation. MPO was 220 ng/ ml at exacerbation and fell significantly to 1 ng/ml at resolution. Levels of IL-8 and IL-5 were elevated during the acute exacerbation and IL-8 concentrations decreased at resolution. In conclusion, airway inflammation can be studied in children with acute asthma by sputum induction. Airway inflammation is present during an acute exacerbation of asthma, and is characterized by infiltration and activation of both eosinophils and neutrophils. The heterogeneity of airway inflammation in acute asthma may influence response to corticosteroid therapy.