Objectives: (1) To elaborate, in preschool children, a valid, discriminating, and responsive measure for assessing asthma severity; and (2) to validate this method in a second cohort of patients. Design: Prospective observational study. Patients/Setting: Children, aged 3 to 6 years (N=199), presenting with acute asthma to a pediatric emergency department. Measurements: The first assessment included a systematic physical exam, O2 saturation, and measurements of airway resistance by forced oscillation (Rfo). A second assessment was obtained, after bronchodilation, in a subset of 129 patients. Results: For the analysis, these patients were subsequently randomized into the test (N=135) and to the validation (N=64) groups. In the test group, the best regression model for predicting acute asthma severity, as measured by the% observed/expected Rfo measurement (or% predicted Rfo), contained five variables. Using these variables, a 10-point measure was elaborated: wheezing(3 points), air entry (3 points), abdominal breathing (1 point), suprasternal retraction (1 point) and O2 saturation (2 points). When tested on the validation group, the resulting score correlated significantly with the% predicted Rfo (r=0.28, P=0.03); the change in score after bronchodilation also correlated significantly with the% improvement in Rfo (r=0.47, P=0.006).Conclusions: This method is a discriminating and responsive means of assessing asthma severity and its response to treatment. It is the first clinical measure, validated in preschool children, against a concurrent objective measure of lung function.