The inflammatory events in the airways at the time of acute respiratory failure from acute severe asthma are poorly understood. To determine the patterns of cellular inflammation in the airways in acute severe asthma, we analyzed tracheal aspirates collected within 12 h of intubation from patients intubated emergently for acute severe asthma (n = 10) and from patients intubated electively for nonpulmonary surgery (n = 14). The number of neutrophils in tracheal aspirates from asthma patients was 10 times higher than normal (4.2 [0.6 to 335.0] [median, range] versus 0.4 [0.009 to 9.4] x 10(6)/ml, p = 0.001), and there was a strong trend for a positive relationship between neutrophil number and duration of intubation (r(s) = 0.64, p = 0.06). Although eosinophil numbers were also significantly higher than normal (0.5 [0.0 to 23.3] versus 0.0 [0.0 to 0.1] x 10(6)/ml, p = 0.003), the numbers of eosinophils were 8-fold less than neutrophils, and there was no significant correlation between eosinophil number and duration of intubation (r(s) = 0.4, p = 0.26). Interleukin-8 (IL-8), a chemoattraction for neutrophils, was 19 times higher than normal in tracheal aspirates from asthmatic patients (75.0 [9.0 to 168.0] versus 4.0 [0.08 to 24.0] ng/ml, p < 0. 05) and correlated significantly with the neutrophil number (r(s) = 0.77, p = 0.03). Furthermore, the IL-8 levels correlated positively with the duration of mechanical ventilation (r(s) = 0.74, p = 0.03). Surprisingly, the number of neutrophils increased significantly during the period of intubation in the asthmatic subjects, possibly because of intravenous corticosteroid treatment. We conclude that neutrophils are the dominant inflammatory leukocyte characterizing airway inflammation in acute severe asthma that requires mechanical ventilation, and that IL-8 is an important mediator of this neutrophilia.