The purpose of this study was to determine whether moderate-intensity exercise training reduces oxidative stress in patients with type 2 diabetes mellitus over 12 months. The patients were divided into 3 groups: aerobic training combined with the use of a fitness center (group A, n = 43), aerobic training only (group B, n = 44), or controls (group C, n = 16). The subjects in groups A and B were instructed to exercise at 50% of peak oxygen uptake for more than 30 minutes on at least 3 days per week over a 12 month period. In addition, the subjects in group A were instructed to use a fitness center and were taught how to perform aerobic training in the indicated manner by certified fitness instructors. We measured the levels of urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) as a parameter of oxidative stress. Serum glycated albumin levels were reduced significantly after 6 and 12 months in groups A and B and after 12 months in group C. Urinary 8-OHdG levels decreased after 12 months in groups A and B, but remained unchanged in group C. There was a significant positive linear association between percentage changes in urinary 8-OHdG and glycated albumin levels over the 12 months. In conclusion, aerobic exercise training improved glycemic control and reduced oxidative stress in patients with type 2 diabetes mellitus. Furthermore, improvement in glycemic control was associated with a reduction in oxidative stress.