Methacholine challenge for diagnosing hyperreactive airways has been valuable in both research and clinical care settings. There are two methods of challenge that are commonly used. With the dosimeter method, five breaths of sequential concentrations of methacholine are inhaled with use of a solenoid valve timing device to standardize the quantity of inhaled methacholine solution. Pulmonary function is assessed after each concentration is inhaled. With the continuous breathing method, each concentration of methacholine is inhaled by tidal volume breathing for 2 min. Pulmonary function is assessed after each concentration. With both methods, a fall in FEV1 of 20% after inhalation of methacholine is diagnostic of hyperreactive airways. Several evaluations demonstrate that pharmacologic bronchial provocation with methacholine is more sensitive than exercise challenge for making the diagnosis of hyperreactive airways. If the allergic athlete complains of exercise-related symptoms, such as cough, dyspnea, or chest pain, and an exercise tolerance test fails to reproduce these symptoms, a methacholine challenge should be performed before the possibility of exercise-induced asthma is dismissed. If the allergic athlete has symptoms of cough or dyspnea that are unrelated to exercise, methacholine challenge is the most sensitive method to evaluate airways hyperreactivity indicative of asthma.