Biochemical evidence of the syndrome of inappropriate antidiuretic hormone secretion (SIADH) was documented in 17 of 24 (71%) children with tuberculous meningitis. Plasma arginine vasopressin concentrations in patients with signs of SIADH were significantly higher (median, 15.44 (range, 1.62 to greater than 24.52) pg/ml; n = 14) than those without (median, 1.91 (range, 0.44 to 4.91) pg/ml; n = 6) (P less than 0.002). Patients who developed evidence of SIADH were older than those who did not (median, 34 (range, 6 to 101) months vs. 10 (range, 6 to 38) months; P less than 0.007). Five patients with and none without died. In 9 patients evidence of SIADH developed only after hospitalization. These patients received a median of 58 (range, 28 to 109) ml/kg/day fluids (n = 7) before developing evidence of SIADH compared with 107 (range, 58 to 146) ml/kg/day received by patients who did not develop SIADH (n = 6) (P = 0.035). SIADH occurs commonly and its presence appears to influence the outcome of tuberculous meningitis in children.