The purpose of this article is to review bronchial reactivity: how it is detected, what are the clinical and pathologic correlates and what treatment modulates it. Is it a good diagnostic test for a disease such as asthma? Is it a reasonable parameter to follow in assessing disease severity and response to treatment?Recent studies on human subjects in the English language medical literature involving bronchial hyperreactivity. In particular all studies were sought that correlated indices of inflammation obtained either by bronchoalveolar lavage or biopsy with degree of bronchial hyperresponsiveness. Pertinent studies reviewing disease association with presence of bronchial hyperreactivity were sought. All studies of treatment with inhaled corticosteroids were reviewed and those in which the subjects could be classified as having either mild or moderately severe asthma on the basis of the clinical information given were used to provide data on correlation of bronchial reactivity and disease severity.The relationship of bronchial reactivity and any disease state is not straightforward. It is not clear what causes hyperreactivity of the airways but there must be other factors besides inflammation as bronchial hyperreactivity can occur without microscopic evidence of inflammation. Hyperresponsiveness does not appear to be as sensitive or specific for the diagnosis of asthma as previously thought, or as closely related to the severity of asthma. Effects of treatment appear to be somewhat independent of their impact on bronchial responsiveness.Airway hyperresponsiveness can be found in many circumstances. Its relationship to symptom production and clinical course in any disease is not clear. Further studies are needed to elucidate the relationship between airways hyperreactivity, asthma, and other lung diseases and the factors that lead to increased bronchial reactivity.