Neutrophil infiltration is a major feature in the pathogenesis of the common cold, and respiratory viral infection is the major cause of asthma exacerbations. The factors regulating the neutrophil influx are unknown. Interleukin-8 (IL-8) is a potent neutrophil chemoattractant, which has been implicated in several inflammatory diseases. In this study, we investigated the presence of IL-8 chemokine in the nasal aspirates of asthmatic children (n = 12) in whom asthma was precipitated by proven viral infection. There were increased IL-8 levels in nasal aspirates from children during the virus-induced asthma exacerbations compared with samples from the same children when they had been asymptomatic for 2 wk (medians 863 and < 20 pg/ml, respectively, p < 0.01). Biological relevance was shown in that IL-8 levels correlate with increased nasal aspirate neutrophil myeloperoxidase levels and there was also a correlation between myeloperoxidase levels and upper respiratory symptom severity. Furthermore, we purified IL-8 from these samples, and demonstrated biological neutrophil chemotactic activity. These are the first in vivo data to suggest an important role for IL-8 in neutrophil influx in proven upper respiratory viral infection associated with asthma exacerbations. We suggest that IL-8 might provide a target for therapeutic intervention in virus-induced respiratory diseases.