Background. Amlodipine, a calcium channel blocker, is reported to have an antioxidative effect in vitro, but whether such an effect occurs clinically is unknown. The purpose of the study was to investigate the effect of amlodipine (5 mg/day) on oxidative stress by measuring urinary 8-iso-prostaglandin F2α (8-iso-PGF2α), an oxidative stress marker, in patients with type 2 diabetes. The anti-oxidative effect of amlodipine was also assessed using the levels of high-sensitivity C-reactive protein (hsCRP) and fibrinogen. Patients and Methods. Seventeen consecutive patients with type 2 diabetes complicated by hypertension were prospectively enrolled in the study. These patients received amlodipine (5 mg/day) over a 3-month period and various markers were measured before and at the end of this period. Results. Urinary 8-iso-PGF2α showed a tendency to decrease, but the change was not statistically significant (P=.1127). The patients were divided into two groups according to the baseline level of urinary 8-iso-PGF2α, and a significant decrease in 8-iso-PGF2α was found in the group (n=9) with a relatively high baseline 8-iso-PGF2α. There were no changes in hsCRP, fibrinogen, and plasminogen activator inhibitor-1. Significant decreases in systolic and diastolic blood pressure were observed (P<.0001, P=.0151, resp.). Conclusion. The results show that amlodipine (5 mg/day) may provide a clinically useful anti-oxidative effect, based on evaluation of urinary 8-iso-PGF2α.