Beta-adrenergic agonists in metered dose inhalers (MDIs) are used extensively in the treatment of asthma. Previously we reported that 23 of 1450 (1.6%) of our asthmatic patients experienced immediate bronchoconstriction after using MDI-albuterol. In this study we investigated immediate bronchoconstriction after use of MDI-metaproterenol and after placebo-inhaler with "inert ingredients" only. Results suggest that those patients who complained of chest tightness or lack of relief following the use of MDI beta-adrenergic agonists are having true bronchoconstriction. The bronchoconstrictive response is most likely caused by the propellants or the other inert ingredients contained in these MDIs.