The lactose that is ingested is hydrolyzed in the intestines to give galactose and glucose. The galactose is then absorbed and the organ that is primarily responsible for its metabolism is the liver. There are three inherited abnormalities of galactose metabolism. The most frequent or classic galactosemia is due to deficiency of galactose-1-phosphate uridyltransferase (GALT), whose clinical manifestations are usually present and are almost always very severe in the neonatal period. Uridine-diphosphate-galactose-4-epimerase deficiency is less common and occurs in two forms: the most unusual clinically resembles congenital galactosemia, while the common form, due to a partial deficiency, is usually benign. Finally, galactokinase deficiency is extremely rare and occurs mainly in certain populations. It causes the development of cataracts, without being accompanied by manifestations of galactose intolerance. Fructose is one of the sweetest components of the human diet. It is found in free form in many vegetables, fruits, and in honey. It combines with glucose to form sucrose in many drinks and preparations. Sorbitol, also present in fruits and vegetables, is converted to fructose in the liver. There are three hereditary abnormalities that affect fructose metabolism. Essential fructosuria is a completely benign abnormality, manifested by the existence of fructosuria after ingestion of fructose. Hereditary fructose intolerance is due to hepatic fructose-1-phosphate (F1P) aldolase deficiency; its clinical manifestations can be diverse. Finally, fructose-1-6-bisphosphatase deficiency is a minor disease of fructose metabolism that, in fact, affects neoglycogenesis.