OBJECTIVE: To evaluate ultrasonographic change of median nerve and flexor tendon in carpal tunnel after local steroid injection in carpal tunnel syndrome (CTS). METHOD: Thirteen patients (24 hands) with CTS diagnosed clinically and electrophysiologically were included. All subjects were examined by ultrasonography with high-resolution linear array transducer and injected with 40 mg of triamcinolone acetonide. The ultrasonography was evaluated with regard to cross-sectional area and the flattening ratio of the median nerve and cross-sectional area of the flexor pollicis longus tenon at pisiform and hamate bone. After 4 weeks, the patients were re-evaluated with electrophysiologic study and ultrasonography. RESULTS: Most patients (11/13) showed improvement of clinical symptoms and electrophysiologic parameters after the local steroid injection. The cross-sectional area of median nerve at pisiform using ultrasonography significantly decreased after the local steroid injection. However, flexor pollicis longus tendon did not show significant change after the injection. CONCLUSION: Local steroid injection in the patients with CTS caused improvement in ultrasonographic findings of median nerve.