A case of a 37 year-old man of congenital diabetes insipidus is reported. He complained of polydipsia, which began from his youth, drinking about 10 L of water every day. He didn't look ill or mentally retarded. In his family, his only brother has shown similar symptoms and his older sister had no symptom. On radiologic study, both kidneys showed severe hydronephrosis and the bladder was markedly distended. Water deprivation test was performed. The result was consistent with diabetes insipidus and the urine osmolality didn't respond to antidiuretic hormone injection, and he was diagnosed as nephrogenic diabetes insipidus. We began to treat him with thiazide and amiloride. The amount of water he's been drinking daily has decreased to 3-4 L per day. We discuss this case with a brief review of literature.