Objective: To investigate the disorders of electrolyte in acute purulent meningitis and meningoencephalitis in children. Methods: A cross-sectional study with convenient sample of 37 pediatric patients with acute purulent meningitis and meningoencephalitis treated in the Pediatric Center of Hue Central Hospital. Results: The average blood sodium concentration is 130 ± 6.1 mmol/l, the lowest concentration is 115 mmol/l, the largest one is 143 mmol/l. Hyponatremia accounted for a high rate of 67.6%, of which the purulent meningitis is 60.0% and the meningoencephalitis is 72.7%. The mild and moderate level of hyponatremia is 76.0%, the severity level is 24.0%. There is no relationship between group disease, signs and symptoms; and outcome with hyponatremia. Other electrolyte disorders are less: slightly hypokalemia in 16.2% and mild hypochloremia in 13.5% of cases. Conclusion: Hyponatremia is a common electrolyte disorder with high ratio and all levels, should be paid attention in monitoring and treating the children with acute purulent meningitis and meningoencephalitis. Key words: hyponatremia, acute CNS infection, purulent meningitis, meningoencephalitis, children.