Galactosaemia is an elevation of blood galactose levels. It may be due to a deficiency of any of the three enzymes of the galactose catabolic pathway: galactose-1-phosphate uridyltransferase (Gal-1-PUT), galactokinase, or UDP-galactose-4-epimerase. Clinically, deficiency of galactose-1-phosphate uridyltransferase (Gal-1-PUT) has become synonymous with classic galactosemia. This autosomal recessive disorder occurs with an incidence of approximately 1:40-60,000 in the general population. The symptoms can be severe in infancy and may lead to death or severe neurological damage if not recognized and treated.

Galactose is a monosaccharide present in many polysaccharides. Clinically, the most important source is the disaccharide lactose. Lactose is the predominant carbohydrate in human and most other animal milk, including cow's milk. Many commercially available infant formulas contain lactose. However, other formulas, such as some soy-based formulas, do not contain lactose. This is critical information to assess in patients as ingestion of galactose is prerequisite to the development of clinical symptoms.