The growth of organic lesions of the central nervous system (CNS) in children, clinical polymorphism, the similarity of clinical and laboratory parameters in inflammatory, demyelinating and oncological diseases necessitate careful differential diagnosis. The clinical case presented in the article confirms the difficulties of differential diagnosis of organic CNS lesion in children, and therefore it is urgent to expand the indications for a brain biopsy, which will allow to timely diagnose correctly, avoid an erroneous diagnostic search and develop adequate tactics.