Noncommunicating cysts of the septum pelludicum are rare and diagnostic experience with CT scan is scanty. They may cause hydrocephalus because of obstruction of Monro's foramina. In this report we describe a young patient with acute Korsakoff's syndrome. The CT scan revealed a cyst of the septum pellucidum without hydrocephalus. Surgery disclosed a noncommunicating cyst compressing structures of the limbic system. After surgery symptoms disappeared.