Current guidelines for asthma care categorize asthma severity based on the frequency of asthma symptoms, medication use, and lung function measures. The objective of this study was to determine whether lung function measures are consistent with levels of asthma severity as defined by the National Asthma Education and Prevention Program/Expert Panel Report 2 Guidelines. Parents of children aged 5-18 years with asthma seen in two outpatient subspecialty clinics completed questionnaires regarding asthma medication use and symptom frequency over the preceding 1 and 4 weeks, respectively. All children performed spirometry. When asthma severity was based on the higher severity of asthma symptom frequency or medication use, asthma was mild intermittent in 6.9% of participants, mild persistent in 27.9%, moderate persistent in 22.4%, and severe persistent in 42.9%. FEV(1) % predicted did not differ by level of asthma severity. FEV(1)/FVC decreased as asthma severity increased (p < 0.0001) and was abnormal in 33% of the participants, and a greater percentage of participants had an abnormal FEV(1)/FVC as asthma severity increased (p = 0.0001). In children, asthma severity classified by symptom frequency and medication usage does not correlate with FEV(1) categories defined by National Asthma Education and Prevention Program Guidelines. FEV(1) is generally normal, even in severe persistent childhood asthma, whereas FEV(1)/FVC declines as asthma severity increases.