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 (partial r2 = 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).(ABSTRACT TRUNCATED AT 250 WORDS)