OBJECTIVE: The purpose of this study was to investigate the effects of moderately extended cycling exercise on oxidative DNA damage (accounted for by urinary 8-hydroxy-2´-deoxyguanosine) in adolescent males and females matched for aerobic capacity. MATERIALS AND METHODS: Twenty-nine aerobically active adolescent males and females matched for peak oxygen uptake (VO2peak) relative to fat free mass (ml/kg FFM/min) participated in this study. Two-hour urinary samples were taken at three time points before (-2-0h), immediately (0-2h) after and 24-26 h after 60 min of cycling exercise at 65%VO2peak, followed by the analysis of urinary 8-OHdG (a potential marker of whole-body DNA damage and repair) determined with high performance liquid chromatography with electrochemical detection. RESULTS: The two-way (time x sex) analysis of variance demonstrated no significant main effects for time, sex or interaction regarding urinary 8-hydroxy-2´-deoxyguanosine level following moderate-intensity endurance exercise. CONCLUSIONS: These results of the present study suggest that no detrimental DNA damage can be observed after moderately prolonged exercise in aerobically fit males and females, potentially because of the enhanced antioxidant defense responses. Furthermore, the endurance-trained adolescent males and females appear to have similar DNA oxidation responses at the whole-body level when normalized to peak oxygen uptake relative to fat free mass.