Human hepatic encephalopathy (HE) is identified by a new noninvasive test, proton magnetic resonance spectroscopy (1H MRS) applied to the brain in a few minutes. Chemical changes identified by 1Η MRS are elevated glutamine, decreased choline and decreased myoinositol. The specific association with HE is proven by clinical studies in patients with cirrhosis, overt and subclinical HE, by the appearance of the same changes after transjugular intrahepatic portasystemic shunt, and by their complete reversal by liver transplantation. The importance of the new marker, myoinositol, may lie in its role as an osmolyte regulating cell volume in the astrocytes. Other roles are also postulated. Progress in the management of both HE and subclinical hepatic encephalopathy may depend upon finding means, short of liver transplantation, which will restore cerebral choline and myoinositol. The finding of identical changes in experimental animals simplifies the search