A 56-year-old woman suffered from severe depression due to Cushing's syndrome with right adrenal adenoma. She had a normal mental state before developing Cushing's syndrome. Because of her depressive state, informed consent for the treatment could not be obtained, and oral administration of 1.5 g/day metyrapone was commenced. After two weeks, her mental state had markedly improved. She subsequently underwent a right adrenalectomy, and metyrapone administration was terminated on the day of surgery. Without modification on the replacement of corticosteroids, the postoperative course was uneventful. Preoperative management of Cushing's syndrome with metyrapone may be useful in cases of severe psychiatric disturbances due to hypercortisolism.