Hypernatremia, a rare condition in newborns, is marked by elevated plasma sodium levels exceeding 150 mEq/L. It is more common in newborns who are exclusively breastfed or with excessive weight loss. Hypernatremic dehydration (HD) presents severe risks, including cerebral edema and other neurological complications. A female newborn, 12 days old (birth weight: 2800g), was brought to the Emergency Department due to a 30% decrease in birth weight and reduced urine output. The newborn was solely breastfed every 2 hours, with a good notion of adequate reflexes and tolerance. Physical examination revealed a skeletal appearance, jaundice, sunken eyes and skin turgor. Blood pressure measurement was unsuccessful, while capillary blood glucose was 96g/dL. A saline bolus (10 mL/Kg) was administered. Venous blood gas analysis showed pH 7.39, lactate 4.3 mmol/L, HCO3- 21.8 mmol/L, and Na+ 180 mEq/L.