We have developed quantitative comparative multiplex dosage analysis to detect altered copy number of regions of the phenylalanine hydroxylase gene. Out of 41 alleles (4% of 1,010 PKU chromosomes) on which a mutation had not been characterized previously, this technique has highlighted two novel mutations: deletions of exon 5 and of exon 6 on a total of eight alleles. Restriction-enzyme digestion of genomic DNA and hybridization to an amplified segment of the phenylalanine hydroxylase (PAH) cDNA probe PAH247 established the size of the deletion in five individuals to be between 700 and 900 bases. We also report somatic mosaicism in the parent of an affected child previously shown to have a deletion spanning exons 5 and 6. Finally, we report a putative duplication of a region encompassing exon 6 in an affected individual. Hum Mutat 21:379–386, 2003. © 2003 Wiley-Liss, Inc.