At the onset, multiple sclerosis (MS) in children is characterized by a phase of active inflammation with extensive demyelination of the white matter of the CNS, which often mimics this nosology with a whole spectrum of inflammatory demyelinating diseases such as ADEM, anti-MOG encephalitis, ONMSD. In order to formalize the diagnostic process, the International Pediatric Multiple Sclerosis Study Group (IPMSSG) proposed diagnostic criteria for the main immune-mediated demyelinating diseases of the CNS in children, which, in theory, should simplify the diagnostic search. However, in practice, the range of nosologies in the course of differential diagnosis is much wider and often it is not possible to establish an unambiguous diagnosis in the early stages. The authors analyze their own clinical observations based on a sample of 100 pediatric patients with a referral diagnosis of «MS?» and describe a clinical case of the onset of highly active MS with an assessment of the dynamics of the clinical and paraclinical course of the disease.