The patient, whowas a non-obese woman with no predisposing conditions of lipomatosis, slowly developed cauda equina syndrome. Spinal magnetic resonance imaging (MRI) presented mass lesion of high intensity on Tlweighted image (WI) and an intermediate signal intensity in T2 WI in the epidural space of SI to coccyges. It has been reported that most idiopathic epidural lipomatosis (IEDL) is observed in obese men, and all cases have involved the thoracic or lumbar region. This is the first report of a patient with cauda equina syndrome caused by idiopathic sacral epidural lipomatosis (EDL). (Internal Medicine 41: 593-594, 2002)