Although racemic albuterol is an effective bronchodilator, regular use has been associated with some loss of bronchodilator potency, decreased protection against bronchoprovocation, increased sensitivity to allergen challenge, and increased sensitivity to some bronchoconstrictor stimuli. In experimental animals racemic albuterol has produced bronchial hyperresponsiveness, which could also be induced by administration of (S)-albuterol. These findings suggest that the pure or homochiral formulation of (R)-albuterol (levalbuterol) might be more effective as a bronchodilator than the racemic form. Single doses of levalbuterol provided more prolonged protection against methacholine challenge than the racemate, whereas (S)-albuterol significantly increased sensitivity to methacholine. In a 4-week study in adults, equivalent amounts of pure levalbuterol provided greater bronchodilation than did similar amounts of levalbuterol given as racemic mixtures. Furthermore, after 4 weeks, the baseline morning FEV(1 ) was lower in those receiving the racemate than in those receiving placebo or levalbuterol. In a single-dose study in children, the same conclusion regarding greater bronchodilation with pure levalbuterol compared with the same amount of levalbuterol in a racemic mixture was confirmed. These studies appear to confirm the greater efficacy of pure levalbuterol over a similar amount in a racemic mixture. This implies a deleterious effect of (S)-albuterol on both the acute bronchodilator response and baseline airway caliber, the exact mechanism of which will require further investigation.