The blood redox status of child athletes is compared with that of age-matched untrained individuals. In the present study, 17 swimmers (10.1 +/- 1.6 years) and 12 non-athletes (9.9 +/- 1.1 years) participated. Reduced glutathione (GSH) was lower by 37% in swimmers compared to non-athletes (P < 0.01), oxidized glutathione (GSSG) was not different and their ratio (GSH/GSSG) was lower by 43% in swimmers compared to non-athletes (P < 0.01). Thiobarbituric acid-reactive substances concentration was higher by 25% in swimmers compared to controls. Catalase exhibited a strong trend toward lower levels in swimmers (P = 0.08). Finally, total antioxidant capacity was found lower by 28% in swimmers compared to controls (P < 0.05). In conclusion, we report that children participating in swimming training exhibit increased oxidative stress and less antioxidant capacity compared to untrained counterparts and suggest that children may be more susceptible to oxidative stress induced by chronic exercise.