Introduction. Myoclonic epilepsy with ragged red fibers (MERRF) is a rare, progressive mitochondrial disease affecting multiple systems, including the central nervous system. Typical MERRF symptoms include: myoclonus, epileptic seizures, ataxia and cognitive decline. In mitochondrial diseases selective cognitive impairment or generalized decline, called mitochondrial dementia, is usually diagnosed.Description of case. We present the case of an 18-year-old patient with progressive neurological symptoms such as multifocal myoclonus, cerebellar syndrome (gait impairment, intention tremor, ataxia and dysmetria). The diagnosis of MERRF was confirmed at the age of 16. Neuropsychological examination showed slowing of verbal learning and deficient spontaneous recall with improvement on recognition as well as low verbal fluency.Discussion. The authors discuss differential diagnosis of mitochondrial diseases (MIDs) in respect to cognitive function impairment and, in particular, to dementia: MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes), KSS (Kearns-Sayre syndrome), NARP syndrome (neuropathy, ataxia, and retinitis pigmentosa and ptosis). The authors emphasize importance of comprehensive neuropsychological assessment in differential diagnosis of MIDs.Conclusion. Mild and selective cognitive impairment was identified. The type and degree of cognitive function impairment is not sufficient to diagnose dementia in this particular case of MERRF. Comprehensive neuropsychological assessment is crucial in MID in order to provide the patient with useful recommendations for education planning.