Airway hyperresponsiveness (AHR) is a fundamental phenomenon in asthma that can explain many aspects of the clinical manifestations of the disease. Several theories on the mechanisms of AHR have been proposed, but the true nature of this problem is yet to be defined. During the past decade, the role of lung inflation in airway physiology and its relationship to AHR have attracted major attention. Deep inspirations are known to exert strong beneficial effects on the airways of healthy humans. These effects appear to be of dual nature: bronchoprotective and bronchodilatory. The bronchoprotective effect of deep inspiration is lost in asthma, even in mild disease. It is also lost in individuals with rhinitis and AHR, but no asthma. Therefore, the loss of bronchoprotection is related to AHR. The bronchodilatory effect of deep inspiration is somewhat reduced in mild asthma and is only lost in severe disease, in the presence of significant airway obstruction. Current research is focused on the elucidation of the physiologic mechanisms behind lung inflation-induced bronchoprotection and bronchodilation and on the causes of their loss. This information could open new horizons in asthma therapy and prevention.