BACKGROUND Autoimmune encephalitis might coexist in patients with autoimmune demyelinating disorders. CASE REPORT We report on a case of a 45-year-old female multiple sclerosis (MS) patient presenting with acute onset short-term memory loss, altered mental status, inflammatory cerebrospinal fluid (CSF) findings and an MRI lesion on the left temporal lobe. An extensive panel for neuronal autoantibodies proved negative. Neuropsychological symptoms gave a prompt response to immunotherapy but nevertheless control MRI showed left hippocampal atrophy. CONCLUSIONS Several recent reports of concurrent emergence of autoimmune encephalitis and MS suggest a common mechanism for these disorders. Since autoimmune encephalitis and MS share certain common CSF and neuroimaging findings, an increased understanding of overlapping autoimmune brain disorders is required to avoid misdiagnosis especially in antibody negative autoimmune encephalitis cases.