Sporadic reports of hypernatremic dehydration in breastfed newborn infants have appeared in medical literature for at least 3 decades. We report the first case of hypernatremic dehydration resulting from inadequate breast-feeding in Korea. A 14-day old baby, born to a mentally retarded mother, was transferred to our hospital with a body weight loss of 460 g since birth(17%) and a serum sodium(Na) level of 179 mEq/L, after initial hydration at another hospital. On admission, a cardiac murmur was heard and an enlarged liver was palpated. Cardiac ultrasonogram revealed ventricular septal defect and ostium secundum atrial septal defect. During hydration, a seizure-like attack developed. Serum Na decreased to 135 mEq/L on the 5th day of admission. Brain ultrasonography and brain magnetic resonance image revealed no remarkable abnormalities. Electroencephalography was normal. She suffered from prerenal azotemia, hyperglycemia and disseminated intravascular coagulation at admission but was treated successfully. Heart failure was also controlled with dobutamine, diuretics and digoxin.