Very few data are available on the structural and functional effects of corticosteroids on peripheral nerve fibers. This paper is addressed to verify possible changes in the functional properties of myelinated Aβ fibers of the median nerve in carpal tunnel syndrome (CTS) after a local injection of triamcinolone acetate. Thirteen subjects affected by mild CTS and 13 controls were selected. Clinical, electrophysiological, and ultrasonographic (cross sectional area: CSA) data of the median nerve were quantified at 0 (pre-injection), and 1 month after triamcinolone injection at wrist. We analyzed the input-output curves constructed by plotting the response amplitude as a function of stimulus intensity. After corticosteroid injection, all patients experienced symptom relief, the median nerve CSA decreased, and the plateau value of the input-output curve was significant higher than before the treatment. Pre injection, we observed a significant inverse correlation between median nerve CSA and plateau value of the input-output curve, which remained significant even after the corticosteroid injection. The Aβ fibers with higher electrical threshold undergo conduction recovery after local corticosteroid injection. In CTS, combined mechanisms might contribute to resolution of symptoms: anti-edema/anti-inflammatory action and direct effect on the electrical properties of the nerve fibers.