In acute meningitis hyponatremia is common and traditionally attributed exclusively to inappropriate water retention. However, the exact mechanisms underlying hyponatremia are unknown. The files of 300 pediatric patients with acute bacterial (n = 190) or aseptic (n = 110) meningitides were retrospectively analyzed. 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. In acute meningitis hyponatremia is not exclusively brought about by inappropriate water retention.