We report a 13-day-old exclusively breastfed neonate, admitted with a history of fever, poor feeding, lethargy and decreased urine output. The mother had history of lactation failure. The neonate had severe hypernatremia, acute renal failure and metabolic acidosis. Renal ultrasound was normal, but the sodium level in mother's milk was 96 mEq/L (normal 7 ± 2 mEq/L). The neonate required peritoneal dialysis on the second day of admission. The biochemical investigations gradually returned to normal and the neonate was discharged on day 12 of admission. We highlight the importance of lactation management and supportive counseling to prevent complications like hypernatremic dehydration due to lactation failure.