Data from studies using the factor analysis technique have shown that asthma appears to be multidimensional and that most of the subjective and objective measures utilized in the assessment of asthma patients represent a much smaller number of underlying dimensions. Additionally, several investigators have emphasized that evaluation of acute asthma is an ongoing process, as the degree and time course of the response to therapy vary considerably between patients. The aim of this study was to examine the usefulness of the most common clinical and objective measures in the evaluation of acute asthma in the emergency department (ED) for predicting the outcome of acute episodes in adults. In an effort to identify variables that can predict the outcome of patients with acute asthma, 184 adults (age 32.4 +/- 11.6 [mean +/- SD]) (analysis sample) who presented to an ED were studied. The inclusion criteria were: (1) age between 18 and 50 years; (2) a peak expiratory flow rate (PEFR) or forced expiratory volume in the first second (FEV1) below 50% of predicted; and (3) no history of chronic cough or cardiac, hepatic, renal, or other medical disease. All patients were treated with salbutamol delivered with metered-dose inhaler (MDI) into a spacer device in 4 puffs actuated at 10-minute intervals and 500 mg of intravenous hydrocortisone. The multivariate statistical technique of discriminant analysis was utilized to develop the index, and after this, a new sample (n = 91), the validation sample, was studied to validate the index. In the analysis sample 163 patients (89%) were discharged (relapse rate within 7 days = 10%) and 21 (11%) were hospitalized (mean duration of hospital stay = 5.75 +/- 2.81 days). The discriminant analysis identified three independent variables that make the greatest contribution in discriminating between the two groups studied: PEFR variation over baseline, PEFR as percent of predicted, and accessory muscle use, all measured at 30 minutes after the beginning of treatment. A multifactorial index using these three variables presented a sensitivity of 0.86, a specificity of 0.96, a positive predictive value of 0.75, and a negative predictive value of 0.98. The validation sample index sensitivity, specificity, and positive and negative predictive values were 0.83, 0.97, 0.83, and 0.97, respectively. In conclusion, a predictive index has been developed for evaluating patients with asthma who present to an ED. This simple and brief three-item index can facilitate an early decision (30 minutes of treatment) to hospitalize patients with severe asthma.