Airway hyperresponsiveness is a characteristic feature of asthma and consists of an increased sensitivity of the airways to an inhaled constrictor agonist, a steeper slope of the dose-response curve, and a greater maximal response to the agonist. Measurements of airway responsiveness are useful in making a diagnosis of asthma, particularly in patients who have symptoms that are consistent with asthma and who have no evidence of airflow obstruction. These tests can be performed quickly, safely, and reproducibly. Certain inhaled stimuli, such as environmental allergens, increase airway inflammation and enhance airway hyperresponsiveness. These changes in airway hyperresponsiveness are of much smaller magnitude than those seen when asthmatic patients with persistent airway hyperresponsiveness are compared to healthy subjects. They are, however, similar to changes occurring in asthmatic patients that are associated with worsening asthma control. The mechanisms of the transient allergen-induced airway hyperresponsiveness are not likely to fully explain the underlying mechanisms of the persistent airway hyperresponsiveness in asthmatic patients.