Classical galactosemia (G/G) is caused by the lack of galactose-1-phosphate uridyltransferase (GALT) activity. A more common clinical variant, Duarte/Classical (D/G) produces partial enzymatic impairment. Although neonatal death due to G/G galactosemia has been largely eliminated by population-based screening and intervention, long-term outcome in some is associated with impaired growth, ovarian failure, dyspraxic speech and neurologic deficits. At least 32 variants in the nucleotide sequence of the GALT gene have been identified and 9 have transferred impaired GALT activity to transformed cells in transfection experiments. We here define the prevalence and biochemical phenotype of two mutations. An A to G transition in exon 6 of the GALT gene converts a predicted glutamine at codon 188 to an arginine (Q188R), and introduces a new HpaII cut site into the gene which enables population screening by polymerase chain reaction. An A to G transition in exon 10 in the GALT gene produces a codon change converting an asparagine to aspartic acid at codon 314 (N314D) and adds an AVA II cut site. We screened a large population for the Q188R and N314D sequence changes to investigate the prevalence of Q188R in G/G galactosemia, the effect of homozygosity for Q188R on outcome, and the prevalence and biochemical phenotype of the N314D sequence change. We found that the Q188R mutation has a prevalence of 62% in a predominately Caucasian population of 107 patients with G/G galactosemia. Homozygosity for Q188R was associated with a poor clinical outcome in a subgroup of these patients. The N314D mutation is associated with the Duarte biochemical phenotype with extraordinary concordance.