A standardized bronchial histamine challenge was performed in 423 patients presenting with 1) asthma, 2) rhinitis, 3) asthma and rhinitis 4) rhinitis with cough or 5) chronic cough alone. Each group was further subdivided into allergic and non-allergic subjects. Twenty healthy subjects served as a control group. Mean histamine thresholds (dose causing 15% decrease in FEV1) of all patient groups differed significantly from controls. There were also significant differences between groups: bronchial responsiveness was higher in allergic than in non-allergic subjects and more marked in asthmatics than in patients with rhinitis or cough. In contrast no differences in bronchial responsiveness to housedust mite or grass pollen could be demonstrated between different groups of allergic patients with asthma and/or rhinitis. Moreover correlations between thresholds for histamine and for these allergens were not significant. This lack of correlation suggests that separate mechanisms are responsible for non-specific and specific bronchial hyperresponsiveness. The presence of asthma in an allergic individual is essentially determined by a high degree of non-specific bronchial hyperresponsiveness.