Obesity is associated with oxidative stress. Endurance training (ET) in healthy individuals increases antioxidant enzyme activity and decreases oxidative stress, whereas its effects on oxidative status in obese humans have yet to be determined. We investigated the effects of obesity and ET on markers of oxidative stress, antioxidant defense, and inflammation. Obese (n=12) and lean (n=12) women underwent 12 weeks of ET with blood, 24-h urine, and muscle biopsies collected prior to and following training for determination of oxidative stress (urinary 8-hydroxy-2-deoxyguanosine and 8-isoprostanes, muscle protein carbonyls, and 4-hydroxynonenal), antioxidant enzyme protein content (muscle CuZnSOD, MnSOD, and catalase), and inflammation (C-reactive protein, leptin, adiponectin, interleukin-6). Obese women had elevated urinary 8-hydroxy-2-deoxyguanosine (P=0.03), muscle protein carbonyls (P=0.03), and 4-hydroxynonenal (P<0.001); serum C-reactive protein (P=0.01); and plasma leptin (P=0.0001) and interleukin-6 (P=0.03). ET decreased urinary 8-hydroxy-2-deoxyguanosine (P=0.006) and 8-isoprostanes (P=0.02) in all subjects and CuZnSOD protein content (P=0.04) in obese women, in the absence of changes in body weight or composition. ET without weight loss decreases systemic oxidative stress, but not markers of inflammation, in obese women.