A 25-year-old Japanese man showed symptoms of common cold and digestive problems for 1 month. He later developed hypoesthesia ascending from the lower extremities and consulted the emergency outpatient department with the chief complaint of generalised dysesthesia. Because of a history of ulcerative colitis, his condition was initially treated as acute aggravation of the disease; however, after admission, his consciousness level gradually deteriorated. Physical findings showed weakened tendon reflexes, and anti-GQ1b antibodies were strongly positive in the cerebrospinal fluid. Therefore, the patient was diagnosed with Bickerstaff's brainstem encephalitis (BBE). Plasmapheresis was performed 8 times, resulting in an improvement of the symptoms; the patient was discharged 1 month later. Campylobacter infections are the main cause of BBE, and its incidence is high among patients with ulcerative colitis. Therefore, in cases where patients with ulcerative colitis develop disturbance in consciousness, BBE should be included in the differential diagnosis.