ADH has been implicated in the development of a positive water balance in thermal injury. However, the association of plasma ADH levels to the hemodynamic response and water and electrolyte balance of early thermal injury has not been defined. Plasma ADH was measured by radioimmunoassay in 13 patients with greater than 15% body surface area burns. In three patients frequent hemodynamic measurements were also made. Large individual variations of ADH were noted, but in general the ADH levels peaked early in the postburn period and declined thereafter. The highest mean ADH value was on day 2 (53.8 = 27 pg/ml); the lowest value on day 7 (13.4 ± 8 pg/ml). Urine electrolytes demonstrated wide variation (Na+, 2 to 236 mEq/L; K+, 2 to 228 mEq/L) as did serum (261–331 mOsm/kg) and urine (557–785 mOsm/kg) osmolalities. No correlation was noted between ADH levels and osmolalities or hemodynamic variables. These data suggest that ADH response following burns is a stress response producing levels beyond the physiologic range for an antidiuretic effect and that the osmolar, hypovolemic, and hypotensive feedback loops are not involved: ADH seems to act as a vasoconstrictor and to have no significant effect on quality or quantity of urine. We conclude that the positive water balance following burns is not ADH mediated.