Neuromyelitis optica spectrum disorder (NMOSD) is a relatively rare but devastating neurological disease characterized by severe episodes of vision loss, spinal cord dysfunction, brain stem and hypothalamus dysfunction. Goal. Epidemiological studies of the prevalence and incidence of patients with orphan disease – NMOSD. Materials and methods. A cross-sectional interval epidemiological study was conducted in 25 subjects of the Russian Federation with different geographical locations. The analysis of information used in the preparation of form No. 12 FSN "Data on number of diseases registered in patients residing in the service area of the medical organization", was selected entries with code ICD-10: G36.0 Neuromyelitis optica (Devik’s disease): data on number of patients with demyelinating diseases of the Central nervous system, G35 – G37, line 7.5. Results. In the Russian Federation, the incidence of rare NMOSD disease is 0.5 [0.45-0.56] per 100,000: the lower limit of the total incidence is taken from the lower value of the 95% confidence interval for subjects not excluded in paragraphs 1 and 2. Conclusions. The predicted number of patients in the Russian Federation at the time of the study and taking into account the limited information on subjects, can range from 660 patients with a low degree of prognosis to 6179 patients with a high degree of prognosis.