Hyponatremia (<135 mmol/L), typically associated with an elevated anti-diuretic hormone level, is common among children admitted with bronchiolitis, pneumonia, or pulmonary exacerbation of cystic fibrosis. The main consequences of acute hyponatremia include cerebral edema and Ayus-Arieff pulmonary edema. A widespread belief is that, in children with pneumonia or bronchiolitis, hyponatremia results from inappropriate anti-diuresis. By contrast, the pathogenic role of extracellular fluid volume depletion or decreased effective circulating blood volume is underscored. Considering the prevalence of hyponatremia, sodium determination is advised on admission in children diagnosed with bronchiolitis, pneumonia, or pulmonary exacerbation of cystic fibrosis. There is no necessity to do anything beyond reassessing the appropriateness of fluid therapy in cases with mild (130-134 mmol/L) hyponatremia. In children with sodium <130 mmol/L, the underlying etiology is sometimes evident from history and physical findings. Given that clinical assessment of fluid volume status is difficult in hyponatremia, further laboratory evaluation is often required in these patients. An increase in sodium level ≤6 mmol/L per day is currently considered the therapeutic goal in all cases. Emergency correction with a 2 mL/kg body weight bolus of 3.0% saline over 10-15 min intravenously is advised in cases with severe symptoms due to hyponatremia and in cases with symptoms, even if mild, due to a rapid-onset (<48 h) of hyponatremia (two additional doses are administered if the patient's condition does not improve).