A linkage between airway hyperreactivity and obesity could partly explain the prevalence of obesity in asthmatics. To test for such a linkage, we analyzed body mass index (BMI), pulmonary function, methacholine bronchial provocation, and asthma severity scores in 216 adolescents (aged 12-18 years), of whom 82 were healthy and 134 were asthmatic. Methacholine provocations in a subgroup of 36 subjects (healthy and asthmatic) enabled us to examine the effects of BMI on dynamic hyperinflation and ventilatory indices during induced bronchospasm. Age- and gender-specific BMI was higher in asthmatics (74 +/- 24%) compared to healthy subjects (61 +/- 28%, P < 0.002). General linear model analysis, in which baseline spirometric results were adjusted for gender, age, race, and height, showed opposing effects of BMI on expiratory flow in controls and asthmatics (P < 0.05), i.e., forced expired volume in 1 sec increased with BMI in controls (P < 0.02), but forced expiratory flow (FEF)(25-75%) decreased with BMI in asthmatics (P < 0.05). However, linear regression analysis showed no effect of BMI on the provocation dose for methacholine (PD(20)) in either controls or asthmatics, and there was no effect of BMI on asthma severity scores. Overweight (BMI >85th percentile) and nonoverweight subjects had similar degrees of dynamic hyperinflation during positive provocations, but overweight subjects had greater decreases in mean inspiratory flow (mean, 28% vs. 9%, P < 0.05). We conclude that our measurements support a relationship between overweight and baseline flow limitation, rather than a relationship between overweight and airways hyperreactivity, in the linkage between overweight and asthma during adolescence.