Hyperphenylalaninemia (Online Mendelian Inheritance in Man database: 261600) is an autosomal recessive disorder mainly due to mutations in the gene for phenylalanine hydroxylase; the most severe form of hyperphenylal-aninemia is classic phenylketonuria. We sequenced the entire gene for phenylalanine hydroxylase in 51 unrelated hyperphenylalaninemia patients from Southern Italy.