Although prostacyclin has been reported to be an effective extracorporeal anticoagulant for intermittent haemofíltration and dialysis treatments, it has been suggested that it is inferior to heparin in preventing clotting in spontaneously driven continuous haemofíltration and/or dialysis circuits. We studied the effectiveness of both heparin and prostacyclin as anticoagulants in a variety of extracorporeal circuits in 17 patients with combined acute hepatic and renal failure who were at risk of haemorrhage. Although there were no differences in the pump-assisted extracorporeal circuits, prostacyclin was found superior to heparin during spontaneously driven continuous arteriovenous haemofíltration and/or dialysis. During some 2,600 h of prostacyclin therapy, there were only 3 episodes of haemorrhage that required blood transfusion compared to 8 major haemorrhages and 2 deaths from intracerebral haemorrhage during 600 h of anticoagulation with heparin. The median filter life was greater with prostacyclin, 60 h (42-72), compared to heparin, 8 h (4-16), p < 0.01. This study suggests that prostacyclin is superior to heparin in maintaining the integrity of a spontaneous arteriovenous extracorporeal circuit in patients at risk of major haemorrha