We report a method by which the contribution of the superior mesenteric vein to the portal blood flow can be evaluated noninvasively. An enteric-coated capsule containing [123I]iodoamphetamine is given by mouth 3h before the examination. The data obtained are treated by computer to calculate the portal shunt index (SI) through the superior mesenteric vein. The SI was higher for more severe liver disorders, Increasing in the order of chronic persistent hepatitis, chronic aggressive hepatitis, and cirrhosis. The SI was higher in cirrhotic patients than in chronic hepatitis patients or healthy volunteers (both, P < 0.0001). The SI was higher in cirrhotic patients with esophageal varices than in such patients without varices (P < 0.05). The SI was higher in cirrhotic patients with ascites than in such patients without ascites (P < 0.001). The SI was higher in cirrhotic patients with encephalopathy than in those without encephalopathy (P < 0.01). Correlation was significant between the SI and classical indicators of functional reserve. This method is clinically useful.