Background: Local steroid injection is used to treat carpal tunnel syndrome (CTS). The aim of this study was to evaluate the clinical and electrophysiological effects of local steroid injection in patients with CTS over a 3-months period. Methods: Twenty-one patients (35 hands) with clinical and electrophysiological evidence of CTS were treated by injection of triamcinolone 40 mg to the carpal tunnel. Visual analog scale (VAS), Boston Carpal Tunnel Questionnaire (BCTQ), rates of paresthesia, night awakening, and electrophysiological studies were used as outcomes. Clinical and electrophysiological assessments were performed before, 1 and 3 months after treatment. Results: Prior to treatment, 86% of patients complained of night awakening. At 1 and 3 months after injection, only 17% and 29% of the patients, respectively, had night awakening (p<0.001). All patients complained of paresthesia before the treatment. This symptom disappeared in 60% and 31% of the patients after 1 and 3 months, respectively (p<0.001). Compared to baseline, both BCTQ and VAS show significant improvement during the 3 months of the study (p<0.005). Although significant improvements in clinical parameters were shown, electrophysiological parameters were not significantly improved at 1 and 3 months. Conclusions: Local corticosteroid injection for the treatment of CTS provides significant improvement in symptoms for 3 months. On the other hand, no significant improvement was observed in electrophysiological parameters.