This study investigated whether postmenopausal women on HRT would experience a greater reduction in oxidative stress after 24 weeks of aerobic exercise training compared to postmenopausal women not on HRT. Plasma thiobarbituric acid reactive substances (TBARS), an indicator of oxidative stress, was measured in 48 previously sedentary postmenopausal women on HRT (n = 21) and not on HRT (n = 27) before and after 24 weeks of aerobic exercise training. Baseline levels of TBARS differed significantly between groups after controlling for age, BMI, and fasting blood glucose (P = 0.03). There was a significant reduction in TBARS after 24 weeks of training in the overall group. When analyzed separately, both postmenopausal women on HRT and those not on HRT had a significant reduction in TBARS; however, there was no significant difference between groups (-0.71 +/- 0.14 nmol/ml in non-HRT users vs. -0.50 +/- 0.16 nmol/ml in HRT users; P = 0.33) even after controlling for age, BMI, and baseline levels of TBARS. Our results showed that aerobic exercise training significantly decreased oxidative stress in postmenopausal women; however, both HRT users and non-HRT users benefited equally.