Lewy body dementia and Creutzfeldt-Jakob disease are recognized on the basis of certain diagnostic criteria. However, common symptoms such as: dementia, extrapyramidal syndrome, psychotic disorders may cause difficulty to make the correct diagnosis especially in the early stage of the disease. Each of these diseases may have atypical onset. The further course and the appearance of other symptoms indicate a proper diagnosis. Electroencephalogram and examination of 14-3-3 proteins in cerebrospinal fluid are helpful in the differential diagnosis. We present a case of a 66-year-old patient initially suspected of Lewy body dementia. On admission, psychomotor retardation, dysarthria, upper extremities dysmetria, extrapyramidal tension in the upper limbs, lower extremities ataxia, slow gait and unstable Romberg test were present. Mini-Mental State Examination (MMSE) score was 24/30. On neuropsychological assessment early stage of dementia was diagnosed. Anxiety-depressive symptoms were observed with periodic visual-auditory components. After less than 3 weeks there was a deterioration of neurological state. Dysarthria and lower limbs ataxia were increased, ataxia of the trunk appeared and psychomotor retardation got worse. There was significant progression of cognitive impairment, therefore complete neuropsychological examination was impossible to perform. MMSE score was 12/30 (12 points less than three months earlier). The course of the disease and additional tests results confirmed the diagnosis of sporadic Creutzfeldt-Jakob disease.