The aims of this study were: to assess the safety of a sputum induction method using inhaled normal saline in children with acute asthma; and to investigate changes in sputum cell counts between acute exacerbations of asthma and its resolution. Ultrasonically nebulized normal saline was used to induce sputum from children (n = 8) presenting with acute asthma within 1 h of arrival and again at least 14 days later, after resolution of the exacerbation. Children received pretreatment with bronchodilator, and peak expiratory flow (PEF) was monitored throughout the procedure. Samples were analysed for total cell count, differential cell counts, and for eosinophils and neutrophils using specific immunochemical stains. Sputum induction was performed without adverse effect in each child with acute asthma. The mean fall in PEF from baseline during sputum induction was 5.3% during the acute attack and 3.4% at resolution. A shorter nebulization time was required to induce sputum in acute asthma than at follow-up (7.8 vs 13.9 min; p = 0.04). During acute asthma, there was an intense cellular infiltrate (mean total cell count 34 x 10(6) cells.mL-1), which resolved after recovery (1.9 x 10(6) cells.mL-1) (p = 0.04). The infiltrate was heterogenous, comprising eosinophils (6.7 x 10(6) cells.mL-1), neutrophils (5.4 x 10(6) cells.mL-1) and mast cells (0.47 x 10(6) cells.mL-1). Resolution of the exacerbation was accompanied by a significant fall in eosinophils and neutrophils (p < or = 0.04). Normal saline induction of sputum can be used to assess airway inflammation in acute asthma. Children with acute asthma have intense airway inflammation that is heterogeneous and involves neutrophils, eosinophils and mast cells.