The importance of inflammation in asthma has been recognized for a long time and recently proved in man and animal models. All inflammatory cells are probably involved in exacerbations of asthma. Neutrophils in particular are present in the airways during and after the spontaneous asthma attacks in man and during asthmatic reactions and airway hyperresponsiveness induced experimentally in man and animals. Depletion of neutrophils prevents these effects and repletion with neutrophils reconstitutes them. Moreover, the supernatant from stimulated human neutrophils causes transient hyperresponsiveness. However, neutrophils are not increased in stable asthmatics with hyperreactive airways and are not involved in airway hyperresponsiveness induced experimentally in some animals (e.g. guinea-pigs). The studies reviewed suggest that neutrophils may be involved in the transient increases of airway responsiveness associated with exacerbations of asthma, but not in the long-lasting hyperresponsiveness of stable asthmatics.