Pretreatment and posttreatment arterial blood gas and pulmonary function testing measurements were prospectively compared as to their ability to assess asthma severity accurately and, thus, predict the outcome in 102 episodes of acute bronchial asthma initially seen in the emergency department. The Pao2, Paco2, or pH was unable to separate these patients requiring admission from those that could be confidently discharged, while the 1-s forced expiratory volume (FEV1) and peak expiratory flow rate (PEFR) did so both before and after treatment. Furthermore, virtually all patients with hypercarbia (Paco2 greater than 42 mm Hg) and/or severe hypoxemia (Pao2 less than 60 mm Hg) had a PEFR below 200 L/min, or an FEV1 below 1.0 L. Thus, selective use of arterial blood gas analysis should substantially decrease both diagnostic cost and patient discomfort without jeopardizing health care.