In an attempt to define guidelines for the assessment of acute asthma, we evaluated 205 patients who presented to the emergency room for treatment of asthma. Of the 205, 120 were successfully treated and discharged from the emergency room, 45 were hospitalized, and 40 were treated and discharged from the emergency room but had relapses within 10 days. A predictive index using a combination of presenting factors was developed: pulse rate greater than or equal to 120 per minute, respiratory rate greater than or equal to 30 per minute, pulsus paradoxus greater than or equal to 18 mm Hg, peak expiratory flow rate less than or equal to 120 liters per minute, moderate to severe dyspnea, accessory-muscle use, and wheezing. The index ranged from 0 to 7, increasing with the severity of symptoms. The index scores of the relapse group (4.9 +/- 1.0) and the admitted group (5.1 +/- 1.0) were both significantly different (p less than 0.001) from that of the successfully treated group (1.6 +/- 1.2). An index of 4 or higher was 95 per cent accurate in predicting the risk of relapse and 96 per cent accurate in predicting the need for hospitalization.