A 21-month-old boy came to our attention because of pneumonia. His weight increased before presentation, and his blood test results showed hyponatremia (116 mEq/L), low plasma osmolarity (241 mOsm/L), and high urine osmolarity (435 mOsm/L). He was treated with 0.9% sodium chloride solution and intravenous furosemide, and sodium levels rose up to 135 mEq/L in 36 hours. No standard treatment is available for severe hyponatremia in children. The use of vaptans in pediatric patients is described in literature, but it lacks evidence about safety and effectiveness. We suggest that furosemide administration plus salt replacement is effective in restoring normal values of plasma sodium concentration in severe euvolemic and hypervolemic hyponatremia.