Creutzfeldt–Jakob disease is a rare, progressive spongiform encephalopathy caused by infectious proteins called prions. It is characterised by rapidly progressive dementia accompanied by cerebellar, visual, extrapyramidal, and pyramidal symptoms, as well as myoclonus and mutism in later stage of the disease. The most common type is sporadic Creutzfeldt– Jakob disease, accounting for 85% of all cases. Treatment of the disease is symptomatic. An important role in making the diagnosis is attributed to the observation of the patient and electroencephalography, showing characteristic cyclical discharges. We present the case of a patient whose first symptoms were psychiatric in nature, and who was diagnosed with Creutzfeldt–Jakob disease based on careful observation, presence of myoclonus, and repeated electroencephalography examinations in which typical 1–2 seconds of sharp and slow wave discharges appeared. By presenting this case of severe progressive encephalopathy, we would like to highlight the fact that even in the age of modern diagnostic methods, electroencephalography, which has been in use for many years, may be crucial in the diagnostic process. We would also like to point out that the initial symptoms of Creutzfeldt–Jakob disease may suggest a psychiatric disorder.