We describe an infant with nephrogenic diabetes insipidus (NDI) with a novel mutation in the arginine vasopressin receptor 2 (AVPR2) gene. A 1-month-old infant showed failure to thrive and hypernatremia. The water deprivation test revealed elevated serum osmolality and low urine osmolality. The patient showed a slight but not significant response to intramuscular injection of arginine vasopressin (AVP). DNA analysis revealed a novel missense mutation involving substitution of proline for leucine at position 173 (P173L), which was reported to be important for stabilizing the hydrogen bond between tyrosine at position 205 and leucine at position 169. This mutation was not detected in 116 ethnic-matched controls. This case, with clinical data including the water deprivation test and P173L mutation, will facilitate understanding the structure and function of the A VPR2.