 The purpose of this study is to determine in a large, double-blind, randomized, prospective pediatric clinical trial whether the use of continuous levalbuterol Xopenex in addition to standard emergency department treatment for acute asthma exacerbations will improve the Forced Expiratory Volume in 1 sec FEV 1 compared to the use of continuous racemic albuterol. The secondary objective is to correlate clinical hospitalization rates and clinical asthma scores with plasma levels of S-albuterol. 