To infer possible mechanisms of acute airway inflammation and mucus hypersecretion in acute severe asthma, we performed cellular and biochemical analysis on sputum from 18 adults with acute severe asthma and compared the results with results of analysis of sputum from 12 adults with cystic fibrosis (CF). We found that in subjects with asthma neutrophils made up more than 75% of sputum cells in 10 samples whereas eosinophils made up more than 75% of cells in only three samples. Fifty percent of the subjects with asthma reported that their asthma exacerbation was precipitated by a respiratory tract infection, and these subjects had a significantly higher percentage of neutrophils in their sputum (85% +/- 6% vs 57% +/- 12%, p = 0.05). In the CF samples neutrophils made up more than 95% and eosinophils less than 1% of cells in all samples analyzed. Analysis of fluid phase chemicals in asthmatic and CF sputum samples showed that despite overall lower mean values of neutrophil elastase (27 +/- 11 micrograms/ml vs 466 +/- 121 micrograms/ml, p = 0.0001) and interleukin-8 (IL-8) (55 +/- 15 ng/ml vs 186 +/- 24 ng/ml, p = 0.0001), some of the asthmatic samples had values for these variables that overlapped those in the CF samples. In addition, the asthmatic samples were distinguished by the presence of higher tryptase (10 +/- 7 U/L vs 0.9 +/- 0.9 U/L, p = 0.0001) and interleukin-6 (1166 +/- 447 ng/ml vs 186 +/- 24 ng/ml; p = 0.0001) levels and by a higher ratio of albumin to mucin-like glycoprotein (0.8 +/- 0.5 vs 0.1 +/- 0.002, p = 0.02). DNA levels were lower in the asthmatic samples (0.5 +/- 0.3 mg/ml vs 3.5 +/- 1.2 mg/ml, p = 0.05). We conclude that neutrophils predominate more frequently than eosinophils as the major inflammatory cell in sputum from patients with asthma in acute exacerbation. We speculate that this may be because respiratory tract infections are a frequent precipitant of acute asthma. In addition, the high IL-8 levels and free neutrophil elastase activity observed in asthmatic sputum suggests that IL-8 may mediate airway neutrophilia in acute asthma and that neutrophil elastase may mediate mucin glycoprotein hypersecretion in acute asthma, as has been proposed for the mucin hypersecretion in CF.