Adenosine, when given by inhalation, initiates the narrowing of airways in subjects with asthma or chronic obstructive pulmonary disease (COPD). The underlying mechanism of this narrowing appears to involve the stimulation of specific mast cell surface adenosine receptors with the subsequent release of mediators and contraction of airway smooth muscle. Although methacholine and histamine have become gold standards as bronchial provocants used to quantify bronchial hyperresponsiveness, the airways response to the indirect stimulus adenosine more closely reflects bronchial inflammation. This distinctive feature of adenosine could be exploited to enable superior diagnostic discrimination between asthma and COPD, allow better monitoring of disease activity and progression, and improve the individual adjustment of long-term asthma management with topical glucocorticosteroids. In this article, we review recent developments in this area of rapidly evolving clinical research, focusing on the putative role of adenosine as a mediator of airway inflammation and as a useful bronchoprovocant in several clinical and research applications.