If accurate predictions of outcome could be made, the emergency care of patients with asthma would be expedited. To evaluate how well initial peak flow determinations predicted hospitalization when used alone and when combined with other clinical variables, we prospectively studied 200 visits for the care of acute asthma. Using discriminant analysis, we selected the variables that best predicted discharge or admission for the first 100 cases. The best predictive variables were initial peak flow, history of treatment in preceding 24 hours, age at onset of asthma, and number of previous hospitalizations for asthma. This combination correctly predicted admission or discharge for 82% of the 200 cases. Despite this overall accuracy, admission was not well predicted. In the first 100 cases, only six of the 18 admissions were correctly predicted, and in the second 100 cases, none of the 15 admissions were correctly predicted, Initial peak flow measurements, even when combined with other variables, cannot predict hospitalization well enough to be substituted for a therapeutic trial.