Plasma amino acids were measured in 18 patients with hepatic encephalopathy on a protein-restricted diet of 20 g or less daily. Plasma aminograms tended to group into two distinct patterns depending on the etiology of the patients' hepatic pathology. Patients with chronic liver disease with superimposed acute insults, i.e., gastrointestinal bleeding, infection, alcoholic hepatitis, had elevated levels of the aromatic amino acids, phenylalanine, tyrosine, and tryptophan, as well as methionine, glutamate, and aspartate, whereas levels of the branched chain amino acids, valine, leucine, and isoleucine, were consistently depressed. Those patients with previously normal livers and acute hepatic necrosis, i.e., "fulminant hepatitis," had grossly elevated levels of all amino acids except the branched chain amino acids, which were normal. Elevations of amino acid levels in this patient group tended to correlate with extent of hepatic necrosis and hence had prognostic significance. Additionally, the different patterns seen in these two groups tend to suggest the indicated therapy as well as predict its efficacy.