OBJECTIVE: In this study, the clinical effectiveness of ultrasound-guided corticosteroid injection "above" versus "below" the median nerve for treatment of patients with mild to moderate carpal tunnel syndrome was compared. DESIGN: This prospective randomized double-blind clinical trial included 44 patients with mild to moderate carpal tunnel syndrome. The subjects were randomly assigned to two groups to receive ultrasound-guided injection of 40 mg of triamcinolone either "above" or "below" the involved median nerve. Outcome measures were the Boston Carpal Tunnel Questionnaire, visual analog scale, electrophysiological tests, and ultrasonographic measurement of the median nerve cross-sectional area at baseline, 6, and 12 wks after the injection. RESULTS: All outcome measures improved significantly in both groups at 6 wks after intervention, and these improvements were persevered up to 12 wks of follow-up (all P values <0.05). However, there was no significant difference in measured outcomes between the two groups. No adverse effects were observed. CONCLUSIONS: Both above and under median nerve ultrasound-guided steroid injection techniques were effective in reducing the symptoms, improving the function, and improving electrodiagnostic and sonographic findings of carpal tunnel syndrome. However, the amount of improvement in the outcomes did not differ between groups, implying that none of technique has the superiority over another.