Autoimmune encephalitis (AE) is a rare disease manifested by rapidly progressive short-term memory loss and other cognitive impairment accompanied by multiple disorders related to the limbic system involvement. The initial symptoms of autoimmune encephalitis may imitate other psychiatric disorders and delay the implementation of an appropriate treatment. The case description of a 15-year-old patient with an initial diagnosis of psychotic disorder has been presented. Because of atypical course of an illness and an ineffective treatment with psychotropic drugs, additional tests were made including serological tests, a cerebrospinal fluid (CSF) analysis and magnetic resonance imaging. Due to the entire clinical picture an autoimmune encephalitis was suspected. The implemented treatment included steroid therapy, intravenous immunoglobulins (IVIG) and plasmapheresis. The treatment regimen was repeated until remission was achieved.