AIM: To evaluate the effects of a portocaval shunt on the decrease of excessive portal flow for the prevention of sinusoidal microcirculatory injury in extremely smallfor-size liver transplantation in pigs.METHODS: The right lateral lobe of pigs, i.e. the 25%of the liver, was transplanted orthotopically. The pigs were divided into two groups: graft without portocaval shunt (n = 11) and graft with portocaval shunt (n=11).Survival rate, portal flow, hepatic arterial flow, and histological findings were investigated.RESULTS: In the group without portocaval shunt, all pigs except one died of liver dysfunction within 24 h after transplantation. In the group with portocaval shunt,eight pigs survived for more than 4 d. The portal flow volumes before and after transplantation in the group without portocaval shunt were 118.2±26.9 mL/min/100 g liver tissue and 270.5±72.9 mL/min/100 g liver tissue,respectively. On the other hand, in the group with portocaval shunt, those volumes were 124.2±27.8 mL/min/100 g liver tissue and 42.7±32.3 mL/min/100 g liver tissue, respectively (P＜0.01). As for histological findings in the group without portocaval shunt, destruction of the sinusoidal lining and bleeding in the peri-portal areas were observed after reperfusion, but these findings were not recognized in the group with portocaval shunt.CONCLUSION: These results suggest that excessive portal flow is attributed to post transplant liver dysfunction after extreme small-for-size liver transplantation caused by sinusoidal microcirculatory injury.