Objective. To evaluate the diagnostic value of MOG-IgG antibodies in multiple sclerosis (MS) and acute disseminated encephalomyelitis (ADEM). Material and methods. Twenty-nine patients with definitive MS according to the criteria of McDonald, 2010 (21 with relapse-remitting MS and 8 with secondary progressive MS), 7 patients with ADEM confirmed by clinical and instrumental data and 27 healthy volunteers were included in the study. Serum MOG-IgG levels were evaluated in all patients. MOG-IgG levels in the cerebrospinal fluid (CSF) were evaluated in 22 cases with MS, 6 cases with ADEM, and 8 healthy volunteers using ELISA. Results and conclusions. Serum MOG-IgG levels are significantly higher in patients with MS and ADEM compared to the control group. No statistically significant differences in the serum MOG-IgG levels were found between MS and ADEM groups. When analyzing the level of MOG-IgG in CSF, no statistically significant between-group differences were obtained. The sensitivity and specificity of the determination of MOG-IgG in serum in patients with MS are 75,8% and 92.59%, respectively. MOG-IgG is detected in serum and CSF in patients with demyelinating diseases that indicates its pathogenetic significance in the development of MS and ADEM. Serum MOG-IgG may be a valuable biomarker for demyelinating diseases. Further research in a large population of patients is required.