Congenital glucose-galactose malabsorption syndrome manifests itself as neonatal-onset severe osmotic diarrhea. This is a rare autosomal recessive disorder caused by damage to the sodiumglucose cotrasporter protein (SGLT1), responsable for the sodium-coupled absorption of these two monosaccharides. We report a case of a healthy, exclusively breast-milk fed newborn, presenting high-output diarrhea, severe malnutrition, and hypernatremia with severe dehydration, who was unresponsive to management with formulas: lactose-free, aminoacid-free, or extensively hydrolyzed. The condition was initially misdiagnosed as diabetes insipidus. With the suspicion of the disease, it was confirmed with administration of a glucose and galactose-free formula as a therapeutic test. Hypernatremia was rapidly reverted, diarrhea disappeared, and progressive nutritional improvement was observed during follow-up. This syndrome has not been previously described in our country, and a high degree of suspicion is required in order to perform the therapeutic test, thus confirming the diagnosis.