RATIONALE: This study was designed to examine the relationships among weight, asthma severity, physical activity, and aerobic fitness in children with asthma. SUBJECTS AND METHODS: Sixty-four asthmatic children 8 to 12 years old (53% female) were assessed while attending a summer asthma camp. Measures included height and weight, spirometry, histamine bronchial provocation challenge, maximal aerobic power, and questionnaires to quantify habitual activity, perceived activity limitations due to asthma, perceived competence in physical activity, and attitudes toward physical activity. Asthma severity was determined from spirometric indices (forced expiratory volume during the first second), degree of airway hyperresponsiveness, and amount of medication prescribed. RESULTS: There was no correlation between asthma severity and aerobic fitness. Only perceived competence at physical activity was found to have a significant correlation with aerobic fitness. Appropriate-weight, overweight, or obese (defined by body mass index) children all had similar results for maximum aerobic power and level of habitual activity. However, overweight or obese children reported greater limitation of physical activity. Their asthma-impairment scores were higher than the scores of appropriate-weight peers, although standard measures of pulmonary function were no different among groups. The higher asthma-severity scores were related to greater medication needs in the overweight or obese children with asthma. CONCLUSIONS: Lower maximum aerobic power in asthmatic children is related more to how capable they perceive themselves than to asthma severity. Overweight asthmatic children experience greater limitation of physical activity and thus are prescribed more medication, although by standard measures of asthma severity, they are very similar to normal-weight peers with asthma. Efforts should be directed at understanding the reasons responsible for reduced exercise tolerance before escalating pharmacologic treatment.