Spirograms were obtained before and after emergency therapy in 85 episodes of acute bronchial asthma in 82 patients. The clinical status of all patients after emergency treatment was reevaluated 48 hours later. Patients could be divided into three groups: I) admissions; II) patients discharged but with later respiratory problems; and III) patients who were discharged and did well. The mean pre- and posttreatment one second forced expiratory volume (FEV1.0) was significantly different among all three groups. FEV1.0 less than or equal to 0.6 liter before treatment, or an FEV1.0 less than or equal to 1.6 liter after emergency treatment, was associated with an unfavorable course. Eighty-eight percent of Group I patients (admissions) had either an initial FEV1.0 less than or equal to 0.6 liter, or a posttreatment FEV1.0 less than or equal to 1.6 liter. Among all patients whose initail FEV1.0 was less than or equal to 0.6 liter, 80% were either admitted or had subsequent respiratory problems; 75% of all patients whose posttreatment FEV1.0 was less than or equal to 1.6 liter were either admitted or developed subsequent respiratory problems. Moreover, 90% of patients who had both a pretreatment FEV1.0 less than or equal to 0.6 liter and a posttreatment FEV1.0 less than or equal to 1.6 liter were admitted or had subsequent significant airway obstruction. We conclude that spirometry can identify asthmatic patients who require admission or who will have significant airway obstruction within 48 hours after discharge from the emergency department.