Exercise-induced arterial hypoxemia (EIAH) is a recognized phenomenon in highly trained adults. Like adult athletes, prepubescent trained children may develop high-level metabolic demand but with a limited lung capacity in comparison with adults. The purpose of this investigation was to search for evidence of EIAH in prepubescent trained children. Twenty-four prepubescent (age: 10.3 +/- 0.2 y) trained children (10.0 +/- 0.7 h of weekly physical activity) performed pulmonary function tests and a graded maximal exercise test on a cycle ergometer. EIAH was defined as a drop of at least 4% from resting level arterial oxygen saturation (Sao(2)) measured by pulse oximetry. EIAH was observed in seven children. Forced vital capacity (FVC), ventilatory response to exercise (Delta(E)/Deltaco(2)), and breathing reserve at maximal exercise were significantly lower, whereas tidal volume relative to FVC was higher in hypoxemic children than in nonhypoxemic children; weekly physical activity and maximal oxygen uptake were similar. Moreover, positive relationships were found between Sao(2) at maximal exercise and breathing reserve (r = 0.56; p < 0.05) or volume relative to FVC (r = 0.70; p < 0.01). EIAH may occur in prepubescent trained children with a relatively low maximal oxygen uptake (42 mL. min(-1). kg(-1)); however, the mechanisms remain unclear and need to be investigated more accurately.