leukoencephalopathy with lesions of the brainstem and spinal cord and increased lactate levels (LBSL) is a hereditary autosomal recessive disease caused by a mutation in the DARS2 gene, with a heterogeneous lesion of the white matter of the brain involving the brainstem and spinal pathways, an increase in lactate in abnormal white matter. Objective. The purpose of the study is to evaluate the features of brain MRI and mitochondrial disorders in adult patients with PE-FMF. Material and methods. An observation of three adult patients from the Moscow region with a hereditary mitochondrial disease of leukoencephalopathy with damage to the brainstem and spinal cord and an increase in lactate level (LBSL) is presented. The diagnosis was confirmed by molecular diagnostics. An MRI examination of the brain was performed on an MRI tomograph with a magnetic field induction of 3.0 T For the cytochemical study of the activity of mitochondrial enzymes in peripheral blood lymphocytes, the method proposed by A.G.E. Pearse modified by R.P. Narcissov. Results. The clinical picture of the disease is similar to multiple sclerosis. MRI of the brain showed more pronounced diffuse changes in the white matter. In all patients, dysfunction of the respiratory chain of mitochondria was noted. Conclusion. Taking into account the data obtained, patients are shown energotropic therapy (idebenone and carnitine preparations). Thus, when doubtful cases in the diagnosis of multiple sclerosis, LBSL should be excluded.