The aim of this study was to determine if prophylactic therapy leads to a reduction in the severity of bronchial hyperresponsiveness (BHR) in subjects with severe asthma. Measurements of bronchial responsiveness to histamine were made in two groups of subjects for periods up to 2 years. Thirteen subjects in the study group took regular medication and used a home monitor of airway function to determine the medication requirements needed to maintain optimal airway function. A control group of eleven subjects was managed with the same drugs but without daily monitoring and without any attempt to keep daily lung function at optimal levels. Subjects in the study group had a 10- to 100-fold decrease in the severity of BHR, which was independent of the improvement in baseline lung function. All but one subject in the study group became symptom free and six were able to maintain the improvement in BHR and symptoms on reduced medication. There was no change in the severity of BHR or in the baseline lung function in the control group. It is concluded that it is possible to reduce the severity of BHR in subjects with severe asthma by the use of pharmacological agents. This reduction in severity appears to require the long-term use of medications, including aerosol corticosteroids, with daily home monitoring to allow adjustment of the amount of treatment required.