A portal hemodynamic study was made in 7 consecutive patients with chronic portal-systemic encephalopathy by percutaneous transhepatic catheterization of the portal vein and injecting contrast medium into the superior mesenteric vein or by superior mesenteric arterial portography in comparison with patients without encephalopathy studied by percutaneous catheterization of these veins. All 7 patients had a large gastro-renal or spleno-renal shunt, and a large proportion of superior mesenteric venous blood was being shunted as estimated from the diameter of the portal and the collateral vein, whereas in nonencephalopathic patients in whom part of the superior mesenteric venous blood was shunting this diversion was much less (P less than 0.001). Only one of the chronic portal-systemic encephalopathic patients had esophageal varices, insignificant in size, and the incidence of esophageal varices was significantly less compared to the 12 nonencephalopathic control patients with portal hypertension who had either a gastro-renal or spleno-renal shunt (P less than 0.05). It is suggested that chronic portal-systemic encephalopathy is a result of a large collateral route shunting a large proportion of the superior mesenteric venous blood into systemic circulation, and that development of such collaterals precludes formation of large esophageal varices.