PIP: The etiology, epidemiology, pathophysiology, and complications, therapy, and prognosis of hypernatremic (hypertonic) dehydration in infants are briefly discussed. The most likely causal condition for hypernatremic states in infants is enteric disease, because the symptoms of diarrhea and vomiting result in water loss and inability to take in water for replenishment. Other causes include dubious feeding practices, diabetes mellitus, diabetes insipidus, and maladroit diagnostic and therapeutic maneuvers, including administration of radiologic contrast medium or hypertonic sodium bicarbonate or mannitol infusions, or the use of salt solutions as an emetic. Epidemiologically, 2 factors are apparent: high saline diet and winter season. The clinical hallmarks of hypernatremic disturbance are relative preservation of circulation and early presence of neurologic symptomatology. Renal tube necrosis is also occasionally encountered. Therapy is rehydration, but the bone of contention is the technique for replacing water in the face of the fact that water administered without electrolyte causes the brain to swell and frequently results in convulsions. The management of hypernatremic dehydration begins with a replenishment phase if neither shock nor apparent anuria is present. The principle is to replenish the body slowly, and 48 hours has been chosen as the target, so that for volume the deficit plus 2 days of ongoing losses should be allocated. However, the sodium and other ion contents are derived solely from the deficit, without factoring the 2-day maintenance period. A recipe for rehydration fluid is presented.