In humans, the absence of galactose-1-phosphate uridyltransferase (GALT) leads to significant neonatal morbidity and mortality which are dependent on galactose ingestion, as well as long-term complications of primary ovarian failure and cognitive dysfunction, which are diet independent. The creation of a knockout mouse model for GALT deficiency was aimed at providing an organism in which metabolic challenges and gene manipulation could address the enigmatic pathophysiologic questions raised by humans with galactosemia. Instead, the mouse represents a biochemical phenotype without evidence of clinical morbidity. The similarities and differences between mice and humans with galactosemia are explored from metabolite, enzyme, and process points of view. The mouse both produces and oxidizes galactose in a manner similar to humans. It differs in brain accumulation of galactitol. Future directions for exploration of this enigmatic condition are discussed.