Phenylketonuria (PKU), an autosomal recessive disorder caused by a deficiency of hepatic phenylalanine hydroxylase (PAH), is clinically very heterogeneous. On molecular level more than 350 mutations in the PAH gene are known to date, which in different genotype combinations could account for biochemical and clinical variability. Mutations located in exon 3 coding for a part of the regulatory domain of the PAH enzyme cause classical PKU, mild PKU, and mild hyperphenylalaninemia (MHP). We describe the phenotypic effects of seven mutations in exon 3 of the PAH gene (R68G, R68S, R71H, S87R, P89S, I95F, and A104D). We propose that mutations located between amino acid positions 71 through 94 cause MHP.