Mouth guards are considered an essential part of equipment for athletes participating in contact sports. The purpose of this study was to evaluate the effect of custom-made mouth guards on the ventilatory gas exchange effects of taekwondo athletes. The subjects were 22 elite athletes aged between 14 and 17 years. To determine the effect of mouth guard use during exercise, oxygen consumption (VO(2)) was measured with a portable gas analysis system while an exercise tolerance test with a shuttle run test protocol was performed. Values with and without mouth guard were compared. Wilcoxon ranks test was used for the statistical analysis. The results show that wearing mouth guards has no significant effect on maximal oxygen uptake (VO(2max)), minute ventilation (VE), tidal volume (VT) and respiratory exchange ratio (RER) while performing maximal exercise (P > 0.05). In conclusion, taekwondo athletes can use custom-made mouth guards without negative effects on their aerobic performance capacity.