Indices of bronchial reactivity were obtained in 11 asthmatic subjects. These were based on the responses of specific airway conductance to timed cumulative doses of methacholine aerosol. The relationship of bronchial reactivity both to initial airway caliber and to current severity of asthma was then assessed. There was no correlation between initial airway caliber and the degree of bronchial reactivity for the group. Furthermore, a subject's bronchial response to methacholine, when expressed as a percentage of initial specific airway conductance, remained stable, even in the presence of moderate fluctuations in initial specific airway conductance. There was also no correlation between bronchial reactivity and severity of asthma for the group. Each subject's bronchial responses remained stable, even in the presence of moderate fluctuations in severity of asthma. These data confirm that a simple relationship between intrinsic bronchial reactivity and the severity of clinical asthma is not apparent.