A number of topical aspects of histamine or methacholine inhalation tests were discussed. First, the reporting of results as the dose delivered to the mouth by the method of aerosol generation and inhalation may allow better interpretation of results between laboratories. However, this requires investigation. Second, the histamine or methacholine dose-response curve differs in asthmatics with a moderate to severe increase in airway responsiveness from asthmatics with a mild increase in responsiveness or nonasthmatics. In the latter groups, the dose-response curve is positioned to the right and has a maximal response plateau. The disappearance of this limited maximal airway narrowing in asthmatics appears to be due to added abnormalities. Third, histamine or methacholine inhalation tests provide a sensitive and specific measure of the presence of variable airflow obstruction (asthma). They are useful to validate the diagnosis when symptoms are suggestive but spirometry is normal. The symptoms of asthma are not specific, and without objective confirmation the diagnosis is frequently misjudged even by the specialist. Finally, airway hyperresponsiveness to histamine or methacholine is not diagnostic of asthma when chronic airflow limitation is present; hyperresponsiveness to isocapnic hyperventilation may be more specific.