Our previous study showed that overuse of the nonparetichand and wrist of the nonparetic side following stroke result in significantly more abnormal on the no nparetic side than on the hemiparetic side in terms of electrophysiologic indice s of median nerve function. The purpose of this study was to evaluate the effect s of the orally administered mecobalamin, an analogue of vitamin B12, for carpal tunnel syndrome (CTS) in the nonparetic side in patients following stroke. In a randomized open label and prospective study of stroke patients, 67 received of 1500 μg mecobalamin daily for 2 years, and the remaining 68 (untreated group) d id not. At baseline, sensory nerve conduction velocity,motor nerve conduction ve locity, sensory nerve action potentials (SNAP) at the wrist, palm-to-wrist dis tal sensory latency, palm-to-wrist SNAP, motor nerve conduction velocity compo und motor action potentials, and distal motor latency of median nerve were signi ficantly more abnormal on the nonparetic side than on the hemiparetic side or in controls.Before the treatment 21 patients (31%) of untreated and 20 patients ( 30%) of treated group met electrophysiologic criteriafor CTS. Sensory impairmen t of the nonparetic side had lessened in the treated group. After 2 years, all e lectrophysiologic indices of nonparetic side were significantly improved in the treated group compared w ith those in the untreated group. The improvement from baseline of electrophysio logic parameters in sensory nerve in the treated group was greater than the impr ovement measured in motor nerve. There were no side effects. Oral mecobalamin tr eatment is a safe and potentially beneficial therapy for CTS in stroke patients. Our previous study showed that overuse of the nonparetichand and wrist of the nonparetic side following stroke result in significantly more abnormal on the no nparetic side than on the hemiparetic side in terms of electrophysiologic indice s of median nerve function. The purpose of this study was to evaluate the effect s of the orally administered mecobalamin, an analogue of vitamin B12, for carpal tunnel syndrome (CTS) in the nonparetic side in patients following stroke. In a randomized open label and prospective study of stroke patients, 67 received of 1500 μg mecobalamin At baseline, sensory nerve conduction velocity, motor nerve conduction ve locity, sensory nerve action potentials (SNAP) at the wrist, palm-to-wrist dis tal sensory latency, palm-to-wrist SNAP, motor nerve conduction velocity compo und motor action potentials, and distal motor latency of median nerve were signi ficantly more more abnormal on the nonparet ic side than on the hemiparetic side or in controls. Before the treatment 21 patients (31%) of untreated and 20 patients (30%) of treated group met electrophysiologic criteria for CTS. Sensory impairmen t of the nonparetic side had lessened in the treated group After 2 years, all e lectrophysiologic indices of nonparetic side were significantly improved in the treated group compared w ith those in the untreated group. The improvement from baseline of electrophysio logic parameters in sensory nerve in the treated group was greater than the impr ovement measured in motor nerve. There were no side effects. Oral mecobalamin tr eatment is a safe and potentially beneficial therapy for CTS in stroke patients.