Exacerbations of asthma requiring emergency treatment are common but, fortunately, asthma is rarely life threatening. In the course of treating acute asthmatics, however, the question of whether to admit the patient often is raised. With the current costs of hospitalization and morbidity associated with the disease, it is advantageous to be able to separate accurately those who require in-hospital treatment from those who can be treated as outpatients. The choice to hospitalize the asthmatic patient must be based on objective findings, including spirometry alone, spirometry plus history and physical signs, or index scoring (Figure). The best method awaits elucidation by prospective study. The most objective indicator of asthma severity is the indirect measurement of airways obstruction by spirometry, either by FEV1.0 or PEFR. FEV1.0 and PEFR yield comparable results. Simple, inexpensive devices for these measurements are available. An FEV1.0 less than 0.8 to 1.0 L (less than 25% predicted in women and men ages 25 to 65 years) or a PEFR less than 100 L/min (less than 20% predicted in women and men ages 25 to 65 years) accurately indicates a severe asthma exacerbation. The percentage predicted is utilized in those individuals who lack average stature. An increase in FEV1.0 to greater than 1.6 to 2.1 L (greater than 60% predicted) or a PEFR improvement to greater than 300 L/min (greater than 60% predicted) after therapy negates the need for hospitalization. In order to gain such improvement the patient may require approximately four to six hours of outpatient treatment. Some will treat patients for longer periods. Evidence of worsening obstruction or impending respiratory failure during this period mandates admission.(ABSTRACT TRUNCATED AT 250 WORDS)