Opsoclonus-myoclonus syndrome (OMS) is a very rare condition with various etiologies (paraneoplastic, parainfectious, toxic, idiopathic, etc.) with an autoimmune pathogenetic mechanism of development. The authors describe the case of OMS in a 41-year-old woman at 37 weeks of gestation, who developed opsoclonus, myoclonus, severe trunk ataxia, tremor and bilateral pyramidal symptoms, inability to sit, stand and walk without support. Differential diagnosis was conducted between virus-induced OMS, rotavirus encephalitis, paraneoplastic syndrome, and demyelinating diseases of the central nervous system. Routine laboratory tests of blood and urine, serological tests of blood and cerebrospinal fluid (CSF) revealed no pathology. Only small lymphocytic pleocytosis and a slight increase in protein were observed in CSF. No pathology was detected during magnetic resonance imaging. On the 40th week of pregnancy (20th day of illness), the patient gave birth to a healthy full-term baby through the birth canal. In view of the most likely autoimmune process triggered by rotavirus infection, intravenous immunosuppressive therapy with methylprednisolone (1000 mg/day №3) was performed, followed by switching to prednisolone per os (60 mg/kg/day), as well as neuroprotective and neurometabolic therapy with cytoflavin. On day 42 of the illness (and on day 20 of the immunosuppressive therapy), a significant positive trend was noted. The patient was discharged on day 56 with light residual elements of opsoclonus and ataxia, and could walk independently without support. Thus, in case of suspected OMS, it is necessary to conduct a mandatory full diagnostic search, especially aimed at exclusion of the paraneoplastic process. And also, given the possibility of recurrence, further outpatient monitoring of these patients should be carried out.