Prior to commencing renal replacement therapy, 8 patients with fulminant hepatic failure and acute renal failure were treated with an infusion of prostacyclin, 5 ng/kg/min, for 30 min, administered directly into the femoral vein. During this period, several adverse effects were noted. There was a reduction in mean arterial blood pressure from a median of 82 (range 65–93) to 67 mm Hg (55–80), p < 0.01; and an increase in intracranial pressure from a median of 14 (6–33) to 17 mm Hg (6–42), p < 0.05; with a consequent reduction in cerebral perfusion pressure from a median of 63 (43–77) to 43 mm Hg (15–74), p < 0.05. There was a reduction in arterial oxygen tension from a median of 19 (13–28) to 16 kPa (12–27), p < 0.05; and no change in cardiac output, from a median of 6.7 (4.9–11.2) to 6.5 1/min/m2 (3.8–11.0), p > 0.05. The administration of prostacyclin into this group of critically ill patients, at risk of death due to cerebral oedema/hypoxia, produced both a reduction in cerebral perfusion pressure and a reduction in total cerebral oxygen delivery.