• Cholesterol, triglyceride, free fatty acid, and phospholipid concentrations were measured in 33 brain, liver, and adipose tissue samples obtained from 17 children who died of an acute encephalopathy associated with liver dysfunction and hyperammonemia (hyperammonemic encephalopathy). Eleven patients had Reye's syndrome, three had acute "toxic" encephalopathy (without fatty liver), two had glycogen storage disease type I, and one had congenital hyperammonemia type 2. Hepatic triglyceride concentrations were markedly increased in Reye's syndrome, but varied from normal to increased concentrations in other hyperammonemic encephalopathies. In contrast, the acute encephalopathy could not be differentiated on the basis of clinical, laboratory, or pathologic features and the brain lipid profiles were remarkably similar among all patients studied.