An infant has a serum sodium of 127 mEq/L, below the normal accepted value of 135 mEq/L. The incidence of hyponatremia is greater than hypernatremia in premature infants. Evidence now shows it is a serious condition in very preterm infants (<33 weeks' gestation), who have large variations of the serum sodium concentration, as they are at risk for poor neuromotor outcome at 2 years. Preterm infants with an increased risk of hyponatremia from sodium restriction show impaired growth and worse neurodevelopment at 10–13 years of age. Hyponatremia is also a risk factor for sensorineural hearing loss, intracranial hemorrhage, and cerebral palsy. Hyponatremia in infants who experienced perinatal birth asphyxia are at risk for an increased mortality.