Relatively little is known regarding the hormonal changes after injury in children. Adult protocols are often applied to children, although the latter often have different physiological responses to trauma. Twenty children with an angulated displaced fracture of the radius and/or ulna (injury severity score 9) were studied prospectively for changes in adrenaline, noradrenaline, cortisol, angiotensin II, arginine vasopressin, urea, electrolytes, and glucose. Two blood samples were taken: one an arrival at the accident and emergency department and one preoperatively several hours later. There were marked increases in adrenaline, noradrenaline, cortisol, and arginine vasopressin above the normal range. Five (25%) cases demonstrated greater early increases in adrenaline than those reported for adult injuries of similar severity. Early hypokalaemia in four cases had corrected towards normal within a few hours, without potassium supplementation.