Circulating sodium abnormalities are associated with an increased risk of morbidity and mortality in patients requiring emergency care. In children with acute gastroenteritis, a major cause of visits in the pediatric emergency department, dysnatremias are observed in one every two cases. Despite common both in infants and adults, the underlying causes of sodium abnormalities in these two age groups are different in most cases. Until the 1980s, hypernatremia was very frequent and hyponatremia uncommon in pediatric subjects affected by acute gastroenteritis. In the following years, hypernatremia almost disappeared, and the prevalence of hyponatremia increased. It is assumed that the resurgence of breastfeeding, formulas having less salt, and early and fast reintroduction of mostly hypotonic fluids, account for this change. New data suggest that climate change could also be a contributing factor. On the other hand, recently published observations point out that hyponatremia might be significantly more frequent in infants affected by bronchiolitis and pyelonephritis than in acute gastroenteritis. The different pathogenetic mechanisms underlying dysnatremia in these conditions have relevant consequences for the nutrition and rehydration management. This talk will deal with these issues and briefly address the approach to acutely ill infants presenting electrolyte imbalances.