The tethered cord syndrome is characterized by sensory and motor disturbances of the lower extremities and incontinence. We report a 12-year-old girl with a cauda equina lipoma and a tethered spinal cord, whose chief complaints were polyuria and polydipsia. She was diagnosed as having nephrogenic diabetes insipidus. This unusual complication of the tethered cord syndrome was most likely due to a hydronephrosis secondary to a neurogenic bladder. Thus, spinal lesions have to be considered in patients with polyuria and polydipsia.