Phenylketonuria (PKU) and benign hyperphenylalaninemia (HPA) result from different combinations of mutations at the locus for phenylalanine hydroxylase (PAH). While some of these mutations show widespread ethnic distribution, others are unique to specific communities. We report here the first point mutation common among Palestinian Arabs. The mutation (IVS2nt1) involves a dinucleotide substitution (Gg-->Aa) at the donor splice site of intron 2 of the PAH gene and abolishes a recognition site of the restriction enzyme MnlI. IVS2nt1 is associated with two PAH polymorphic haplotypes, 7 and 42. Homozygotes for this mutation are affected with severe, classical PKU. Compound heterozygotes carrying the IVS2nt1 allele and one of several other yet unknown mutations show different degrees of benign HPA.