The purpose of this study was to test the influence of custom-made mouth guards on strength and anaerobic performance of taekwondo athletes. The study included 21 (11 male and 10 female) trained subjects participating in taekwondo. Anaerobic power and anaerobic capacity, isokinetic quadriceps and hamstring strength, handgrip strength, isometric lower extremity and back strength, 20 m sprint time, squat and counter movement jumping height were measured in two randomized conditions: with or without custom-made (CM) mouth guards. No significant differences were observed between the two conditions (with or without CM mouth guards) in 20 m sprint time, jumping tests, handgrip strength, isometric leg or back strength. On the other hand, peak power and average power in Wingate Anaerobic Test and Hamstring Isokinetic Peak Torque significantly increased as a result of wearing mouth guard (P < 0.05). In conclusion, we can suggest that taekwondo athletes can use CM mouth guards without any negative effects on their strength and anaerobic performance. © 2009 John Wiley & Sons A/S.