A congenitally patent ductus venosus is the rarest form of portosystemic shunt. Although it occurs in the absence of serious parenchymal liver disease, it may be associated with hepatic encephalopathy. This case report describes a 49-year-old man with a patent ductus venosus who presented with recurrent bouts of encephalopathy but few other features of chronic liver disease. Ligation of the shunt was followed by life-threatening complications. The portal venous system must be carefully assessed before closure of a patent ductus venosus is attempted.