Leigh syndrome is a neurodegenerative disease caused by reduced energy production in the mitochondria, and is caused by mutations in one of over 100 genes in the nuclear or mitochondrial genome. It manifests itself in early childhood with subacute necrotizing encephalopathy characterized by impaired consciousness, psychomotor regression, dystonia, bulbar paralysis, and respiratory failure. The neuroimaging feature is bilateral symmetrical lesions of the basal ganglia, brainstem and cerebellum. The aim of this thesis was to describe the clinical, neuroimaging and genetic characteristics of patients with Leigh syndrome treated in the Department of Genetics and Metabolic Diseases of the Pediatric Clinic of University Hospital Centre Zagreb between 2012 and 2022. This retrospective study included 18 patients diagnosed with Leigh syndrome. The hospital archive, hospital information system and outpatient records were used as data sources. The diagnosis was established on the basis of the clinical picture, biochemical and neuroimaging findings, and in most subjects it was confirmed by genetic analysis. The median age at diagnosis was 11 months. In 44,4% of patients, a mutation was found in one or both parents. The most common provoking factors were respiratory infection and acute gastroenterocolitis. The most common clinical signs of encephalopathic crisis were loss of consciousness, epileptic seizures, weakness and hypotonia. The most significant biochemical features were increased concentrations of lactate and alanine in the blood and cerebrospinal fluid and pathological excretion of organic acids in the urine. Neuroimaging revealed hyperintensities in the T2/FLAIR weighted images in the area of the putamen, globus pallidus, mesencephalon, pons and subcortical white matter. Patients were treated with the so-called mitochondrial cocktail (combination of coenzyme Q;,, biotin, thiamin, riboflavin and antioxidants, usually vitamin C). Palliative care is the backbone of care for patients with Leigh syndrome. Five patients died as a result of Leigh syndrome, and 13 showed signs of severe psychomotor retardation. The results of this study confirm that Leigh syndrome is a severe neurodegenerative disease that leads to psychomotor retardation and premature death.