As part of a prospective study of children with bacterial meningitis we analyzed in 36 patients of our hospital the fluid balance on admission and during the first three days of treatment. On admission 10 of them (28%) had inappropriate antidiuretic hormone secretion SIADH, 10 (28%) hypo-osmolal and 10 (28%) iso-osmolal contraction. Six patients (17%) had no clear fluid disorder. The patients with SIADH had significantly lower mean serum NA+ (127 vs. 132 mEq/l, p less than 0.01) and higher mean urine Na+ (111 vs. 26 mEq/l, p less than 0.01) concentration as well as higher mean urinary Na+/K+ ratio (2.23 vs. 0.365, p less than 0.005) than the patients with hypo-osmolal contraction. They also tended to be younger and have a shorter history of fever. The patients with SIADH had a less strict fluid restriction than the patients with hypo-osmolal contraction, and their fluid balance normalized more slowly. Our findings support initial water restriction for all children with bacterial meningitis.