Background: Multiple sclerosis (MS) and neuromyelitis optica (NMO) are both demyelinating disorders and oxidative stress is suggested to have a role in their pathogenesis. Glucose-6-phosphate dehydrogenase (G6PD) produces nicotinamide adenine dinucleotide phosphate (NADPH) via the pentose phosphate pathway. NADPH is not only involved in the synthesis of fatty acids necessary for myelination, but also it is involved in the defense against oxidative stress. Prescribing supplementary vitamin D as a part of the MS treatment plan can increase G6PD gene expression. The aim of this study was to determine the serum level of G6PD in patients with MS and NMO and its relationship with vitamin D, since it is yet to be explored thoroughly.Methods: In this case-control study, subjects were divided into three experimental and control groups. The experimental groups comprised 50 patients with relapsing-remitting MS (RRMS) who had a history of vitamin D consumption, 50 newly-diagnosed MS patients, and 50 patients with NMO. Control group included 65 healthy individuals. Serum level of G6PD was measured and compared among these groups.Results: No significant difference was seen between the G6PD level in patients with MS and NMO, but it should be noted that this level was significantly lower than the healthy group. G6PD serum level was significantly higher in patients with MS who had previously consumed supplementary vitamin D compared to those who had not.Conclusion: G6PD deficiency is observed in patients with MS and NMO. Also, supplementary vitamin D may induce favorable results on the G6PD level.