Corticosteroid treatment is frequently associated with psychiatric disturbances. These adverse effects are unusual with low dose of corticosteroid. We describe a patient who rapidly developed a steroid-induced psychosis with very low dose of prednisolone. A 48-year-old woman of Sheehan's syndrome was admitted to hospital with insomnia, euphoric moods and visual hallucinations. She had taken prednisolone (10 mg in the morning and 5 mg at night) for 6 days before admission. These symptoms appeared after first dose of prednisolone. A diagnosis of acute psychosis was made. After improvement of acute psychosis with discontinuation, re-administration of prednisolone at a dose of 2.5 mg per day gave rise to agitation and insomnia. She recovered completely by gradual dosage increase of short-acting corticosteroid after the discontinuation of prednisolone.