An unusual congenital portosystemic shunt was identified in one dog and two cats with clinical signs and laboratory evidence of hepatic dysfunction. In all the animals, the abnormal vessel arose from the portal system between the left medial and quadrate liver lobes and travelled within the falciform fat, exiting the abdomen through the caudal ventral left diaphragm. The intrathoracic course of these vessels was not established. The anatomical location of this anomalous vessel may have hindered attempts at ultrasonographic identification since it was not visualised before surgery in any of the animals. In addition, while the anatomical location of the vessel may facilitate rapid identification and surgical attenuation, it could predispose the vessel to trauma during the coeliotomy approach. It is hypothesised that this form of portosystemic communication results from failure of a portion of the left umbilical vein to degenerate during embryogenesis. This is in contrast to other forms of congenital extrahepatic portosystemic shunt that are presumed to be developmental errors resulting in an abnormal communication between the embryonic vitelline and cardinal venous systems. The prognosis for animals with the vascular anomaly reported here is probably similar to that for animals with other forms of congenital portosystemic shunt.