Changes in body water and electrolytes were studied in 30 children, 2 months to 5 years old, with acute meningitis. Total body water (TBW), extracellular water (ECW) and urinary and serum sodium and osmolality were estimated on the day of hospitalization (Day 1) and after recovery/Day 10. Thirty age- and sex-matched children served as their controls. The TBW (683 ± 63 (mean ± SD) ml/kg) as well as ECW (311 ± 76 ml/kg) was significantly higher in children with meningitis on Day 1 than in the controls (TBW 642 ± 49 ml/kg, ECW 271 ± 62 ml/kg) (P< 0.01), whereas the values after recovery (TBW 643 ± 48 ml/kg, ECW 272 ± 63 ml/kg) were similar to those of controls. Thus on the average a child with acute meningitis had excess body water, all of which was in the ECW compartment. The mean ECW excess was 33 ± 32 ml/kg. Twenty-four of 27 surviving children had higher TBW and ECW at the time of admission compared with the values after recovery.On multiple regression analysis, the only significant determinant of ECW excess was severity of the illness (partialr2= 0.62). The ECW excess was 70 ± 8 ml/kg in severely ill, 50 ± 32 ml/kg in moderately ill and only 12 ± 18 ml/kg in mildly ill children (P< 0.01). Those who had complications or sequelae had much higher ECW (49 ± 26 ml/kg) than those who recovered completely (17 ± 27 ml/kg). Syndrome of inappropriate secretion of antidiuretic hormone according to set criteria was diagnosed in 14 children; each had an ECW excess of >45 ml/kg. The ECW excess had a very high negative correlation with serum sodium concentration (r= −0.82). On multiple regression analysis ECW excess was the only significant determinant of serum sodium concentration (partialr2= 0.47). From our data it appears reasonable to consider fluid restriction in those children with acute meningitis who have the syndrome of inappropriate secretion of antidiuretic hormone or hyponatremia. However, further studies are needed to evaluate the impact of fluid restriction on the outcome of acute meningitis.