Classic galactosemia is an autosomal recessive disorder caused by the deficiency of galactose 1-phosphate uridyltransferase (GALT). Although the potentially lethal, neonatal hepatotoxic syndrome is prevented by newborn screening and galactose restriction, long-term outcome for older patients with galactosemia remains problematic. After the cloning and sequencing of the GALT gene, more than 130 mutations in the GALT gene have been associated with GALT deficiency; this review relates them to function and clinical outcome. Two common mutations, Q188R and K285N, account for more than 70% of G alleles in the white population and are associated with classic galactosemia and impaired GALT function. In the black population, S135L accounts for 62% of the alleles causing galactosemia and is associated with good outcomes. A large 5 kb deletion in the GALT gene is found in Ashkenazim Jews. The Duarte galactosemia variant is caused by N314D. Homozygosity for N314D reduces GALT activity to 50%. When either E203K or a 1721C-->T transition (Los Angeles variant) are present in cis with N314D, GALT activity reverts to normal. In this review, we discuss the structural biology of these mutations as they affect both the GALT enzyme and patient outcome.