Classical galactosemia is caused by one common missense mutation (Q188R) and by several rare mutations in the galactose-1-phosphate uridyltransferase (GALT) gene. The most common variant of GALT, the Duarte variant, occurs as two types, Duarte-1 (D-1) and Duarte-2 (D-2), both of which carry the sequence change N314D. D-1 increases, whereas D-2 decreases GALT activity. To study the molecular genetics of classical and Duarte galactosemia, we analyzed the GALT mutations in 30 families with classical galactosemia, in 10 families with the D-2 variant and in 3 individuals carrying the D-1 allele by denaturing gradient gel electrophoresis (DGGE). DGGE detected 59 of the 60 classical galactosemia alleles. Q188R accounted for 60%, K285N accounted for 28% of these alleles. Eight novel candidate galactosemia mutations were found. On all D-2 alleles N314D occurred in cis with two intronic sequence changes, on the D-1 alleles in cis with a neutral mutation in exon 7. We conclude that the mutations causing galactosemia are highly heterogeneous and that K285N is a second common galactosemia mutation in our population.