OBJECTIVE: To determine whether lung function alters asthma severity based on symptom history in asthmatic adolescents. DESIGN: Data on asthma symptoms and lung function were collected from adolescents randomly selected from the general population. SETTING: Five schools from the central Wellington, New Zealand, area during 2003 to 2005. PARTICIPANTS: Two hundred twenty-four secondary school students aged 13 to 17 years (asthmatic, 118; nonasthmatic, 106). MAIN EXPOSURES: Asthma questionnaire and lung function testing. MAIN OUTCOME MEASURES: Distribution of asthmatic adolescents in each severity class based on symptoms, lung function, or a combination of both. RESULTS: Median values for all spirometric parameters for asthmatic adolescents, apart from forced expiratory volume in the first second of expiration (FEV(1))/forced vital capacity (FVC), were in the normal range. Distribution of severity (based on symptoms and beta(2)-agonist use with adjustment for regular inhaled corticosteroid use) was 48.3%, mild; 28.8%, moderate; and 22.9%, severe asthma. For severity based on percentages of predicted FEV(1) and predicted forced expiratory flow, midexpiratory phase (FEF(25%-75%)) and FEV(1)/FVC, the percentages were 89.8%, 86.4%, and 63.5%, mild; 9.3%, 10.2%, and 18.6%, moderate; and 0.9%, 3.4%, and 17.8%, severe asthma, respectively. When percentages of predicted FEV(1) or predicted FEF(25%-75%) or FEV(1)/FVC were added to symptom severity, 6.8%, 5.1%, and 16.9% of asthmatic adolescents were reclassified into another severity group, respectively. CONCLUSIONS: The majority of asthmatic adolescents have normal lung function despite experiencing significant asthma symptoms. Adding FEV(1)/FVC to symptom history changes the distribution of severity; however, both percentages of predicted FEV(1) and FEF(25%-75%) have little added effect in assessing asthma severity in adolescents.