A 25-year-old woman suffered from hyperemesis gravidarum when she was seven weeks pregnant. Since her vomiting continued, she received intravenous dextrose and electrolytes without thiamine in a hospital. One month later, she developed gait disturbance, followed by confusion and dysarthria. On admission to our department, she was confusional and had ataxic dysarthria. Spontaneous and gaze evoked nystagmus was present. Limb coordination was bilaterally ataxic. Based on her clinical course and symptoms, she was diagnosed as having Wernicke's encephalopathy. From the admission day, intravenous infusion of vitamin B1 (600 mg/day) was started. A few days later, her consciousness and limb ataxia began to improve. However, truncal ataxia and polyneuropathy became evident. Eight weeks after onset, she developed Korsakoff's psychosis such as anterograde and retrograde amnesia, disorientation and confabulation. We administered large amounts of corticosteroid (methylprednisolone 500 mg/day) in order to reduce brain edema or stabilize the impaired blood-brain barrier. Soon after, her psychosis began to improve gradually. She recovered remarkably from the psychosis, but she was left with persistent nystagmus, mild ataxic gait and polyneuropathy. The present case suggests that corticosteroid may have the beneficial effect on Wernicke-Korsakoff syndrome.