The purpose of this study was to compare the prevalence and implications of expiratory flow limitation (EFL) during exercise in boys and girls. Forty healthy, prepubescent boys (B; n=20) and girls (G; n=20) were tested. Subjects completed pulmonary function tests and an incremental cycle maximal oxygen uptake (VO2max) test. EFL was recorded at the end of each exercise stage using the % tidal volume overlap method. Ventilatory and metabolic data were recorded throughout exercise. Arterial oxygen saturation (SpO2) was determined via pulse oximetry. Body composition was determined using dual-energy X-ray absorptiometry. There were no differences (P>0.05) in height, weight, or body composition between boys and girls. At rest, boys had significantly higher lung volumes (total lung capacity, B=2.6+/-0.5 liters, G=2.1+/-0.5 liters) and peak expiratory flow rates (B=3.6+/-0.6 l/s; G=1.6+/-0.3 l/s). Boys also had significantly higher VO2max (B=46.9+/-5.9 ml.kg lean body mass(-1).min(-1), G=41.7+/-6.6 ml.kg lean body mass(-1).min(-1)) and maximal ventilation (B=49.8+/-8.8 l/min, G=41.2+/-8.3 l/min) compared with girls. There were no sex differences (P>0.05) at VO2max in VE /VCO2, end-tidal PCO2, heart rate, respiratory exchange ratio, or SpO2. The prevalence (B=19/20 vs. G=18/20) and severity (B=58+/-7% vs. G=43+/-8% tidal volume) of EFL was not significantly different in boys compared with girls at VO2max. A significant relationship existed between % EFL at VO2max and the change in end-expiratory lung volume from rest to maximal exercise in boys (r=0.77) and girls (r=0.75). In summary, our data suggests that EFL is highly and equally prevalent in prepubescent boys and girls during heavy exercise, which led to an increased end-expiratory lung volume but not to decreases in arterial oxygen saturation.