Sixty-seven episodes of acute asthma were treated in an emergency room. The characteristics of the attacks and subsequent course were then analyzed to determine criteria that could be used for an early decision in regard to the need for hospitalization. Attacks that were not successfully treated in the emergency room were most often characterized by very severe obstruction and a poor response to an initial injection of epinphrine. It is suggested that severely obstructed patients (peak flow less than 16% of predicted) whose peak flow remains less than 60 liters/min, or who exhibit a less than 16% improvement following 0.3 ml epinephrine, be promptly admitted.