The molecular analysis of the human phenylalanine hydroxylase (PAH) gene in Mexican phenylketonuric (PKU) patients is described. We analyzed the restriction fragment length polymorphism (RFLP) haplotypes of five probands and ten non-affected relatives, belonging to four unrelated PKU families. Twenty-nine alleles were typified, corresponding to 12 different haplotypes. Eight RFLP haplotypes corresponded to those described in other populations, while the remaining were unreported haplotypes, appearing both on normal and PKU chromosomes. Using the polymerase chain reaction (PCR) and the allele-specific oligonucleotide assay (ASO), we also screened for the IVS10 mutation, one of the most common PAH gene mutations in Mediterranean countries. Forty-two percent of the PKU chromosomes analyzed bore the IVS10 mutation, although it was present in the heterozygous state in all cases. Our data show an important genetic heterogeneity at the PAH locus in the Mexican population, and report the genetic influence of the Spanish immigration to the American continent.