Introduction: In pediatric and adolescent population, autoimmune encephalitis (AE) may present with a wide variety of symptoms including cognitive regression accompanied with loss of language skills. Despite its high prevalence in AE, linguistic functions have not been investigated in extensive detail. Case: A 12-year-old girl with no significant premorbid history and normal school performance presented with fever, hypersomnia, nocturnal myoclonus and behavioral changes. Although neurological examination was normal, psychiatric evaluation revealed euphoria, mild irritability and visual hallucinations. Cranial MRI was normal, whereas cerebrospinal fluid (CSF) analysis showed elevated protein concentration and lymphocyte count and electroencephalogram (EEG) showed diffuse slow waves. A panel for anti-neuronal antibodies demonstrated glutamic acid decarboxylase (GAD) antibodies in the serum. Following immunotherapy, all neurological and behavioral symptoms vanished. However, the patient suffered from significant worsening of school performance. Psychiatric evaluation revealed severe depression. Assessment of intelligence done on the 10th and 18th month of follow-up yielded significantly low scores at mental retardation level. Linguistic assessment showed significant impairment in all domains but especially in semantics. Conclusion: Our case emphasizes the fact that AE may cause permanent cognitive dysfunction and language impairment even in patients with normal MRI/neurological examination findings and relatively mild treatment-responsive disease course.