The inhalation challenge with lysine-aspirin (L-ASA) using the dosimeter method allows the construction of a dose-response curve and the quantitative estimation of airway responsiveness to the drug. We assessed the modifications of airway responsiveness to methacholine in four groups of subjects: aspirin-sensitive asthmatics, aspirin-sensitive subjects with urticaria/angioedema, subjects with an equivocal history of aspirin intolerance and normal control subjects. The L-ASA challenge was positive in all aspirin-sensitive asthmatics. The pattern of bronchial response to the challenge was different from that observed after challenge with allergens or occupational sensitizers. The main difference was found in the recovery from induced bronchoconstriction. The recovery lasted from 3 to 6-8 hours, and a peculiar dose-response curve was obtained that we call "early prolonged reaction". In five of 18 ASA-sensitive subjects there was a significant increase in airway responsiveness. Airway responsiveness was normal in aspirin-sensitive nonasthmatic subjects and in the other two groups studied. We conclude that L-ASA inhalation challenge may increase bronchial hyperresponsiveness in some ASA-sensitive asthmatics. This presence of enhanced bronchial hyperesponsiveness seems to be a marker with which to distinguish ASA-sensitive asthmatics from ASA-sensitive subjects with urticaria/angioedema.