We report the successful use of i.v. magnesium sulphate to control life-threatening autonomic hyper-reflexia associated with chronic spinal cord injury in the intensive care environment. A 37-yr-old, male was admitted to the intensive care unit with a diagnosis of septic shock and acute renal failure secondary to pyelonephritis. He had been found unresponsive at home following a 2-day history of pyrexia and purulent discharge from his suprapubic catheter. He had sustained a T5 spinal cord transection 20 yr previously. Initial management included assisted ventilation, fluid resuscitation, vasopressor support, and continuous veno-venous haemofiltration. The sepsis was treated with antibiotic therapy and percutaneous nephrostomy drainage of the pyonephrosis. On the fifth day, the patient developed profuse diarrhoea. This was associated with paroxysms of systemic hypertension and diaphoresis, his arterial pressure rising on occasion to 240/140 mm Hg. A diagnosis of autonomic hyper-reflexia was made and a bolus dose of magnesium sulphate 5 g was administered over 15 min followed by an infusion of 1-2 g h(-1). There was an almost immediate decrease in the severity and frequency of the hypertensive episodes. There were no adverse cardiac effects associated with the administration of magnesium, only a slight decrease in minute ventilation as the plasma level approached the upper end of the therapeutic range (2-4 mmol litre(-1)). In view of the beneficial effects observed in this case we advocate further research into the use of magnesium sulphate in the treatment or prevention of autonomic hyper-reflexia secondary to chronic spinal cord injury in the intensive care unit.