BACKGROUND: In acute meningitis hyponatremia is common and traditionally attributed exclusively to inappropriate water retention. However, the exact mechanisms underlying hyponatremia are unknown. METHODS: The files of 300 pediatric patients with acute bacterial (n = 190) or aseptic (n = 110) meningitides were retrospectively analyzed. RESULTS: The plasma sodium level ranged from 122 to 148 mmol/l and was low (<133 mmol/l) in 97 patients. Fluid volume contraction was significantly more pronounced in hyponatremia (median 6.0. 10(-2)) than in normonatremia (median 2.0. 10(-2)). The fractional sodium excretion was less than 1.00. 10(-2) in the 26 hyponatremic children with this measurement. CONCLUSION: In acute meningitis hyponatremia is not exclusively brought about by inappropriate water retention.