A 52-year-old male patient with RRMS presented with left hemiparesis.The patient was diagnosed with RRMS in 2007.He received interferon beta-1a,natalizumab,fingolimod,rituximab,and ocrelizumab treatments that were considered to be inefficient,and therefore switching to alemtuzumab was decided.Before starting alemtuzumab,his neurological examination revealed mild dysarthria,mild bilateral upper extremity paresis(BMRC 4/5),prominent spastic paraparesis(BMRC 2/5),and walking difficulty(EDSS 6,5).After four days of intravenous alemtuzumab administration(12 mg/day),the patient received ertapenem due to a urinary tract infection caused by E. coli.The final dose of alemtuzumab was delivered after two weeks of antibiotic treatment.After 51 days of the last alemtuzumab dose,the patient complained of increased numbness and weakness in both lower and upper extremities.His routine blood and urine tests were within normal limits.Eighteen days after the onset of the symptoms,we found left peripheral facial palsy,quadriparesis with neck extensor muscle weakness,hypotonia,and areflexia with an EDSS of 8,5.The patient’s brain and cervical spinal MRIs revealed no new lesions.His electromyography revealed an acute inflammatory demyelinating polyneuropathy(AIDP) with predominant motor fiber involvement.The patient’s cerebrospinal fluid (CSF) analysis revealed an elevated protein level(88 mg/dl,normal range:15–45) without any cells.Serum and CSF anti-ganglioside antibodies were negative.With the diagnosis of Guillain-Barré syndrome(GBS),he was treated with intravenous immunoglobulin(2 g/kg/day) for five days.Symptoms and signs were improved gradually,and the patient was discharged ten days later with an EDSS score of 7.0.