We here describe the detection of a de novo mutation in the phenylalanine hydroxylase gene in a Norwegian phenylketonuria (PKU) patient. This novel mutation, M1I, disrupts the start codon of the gene by a G to A transition. The compound heterozygote genotype (IVS-12/M1I) of this patient predicts that no phenylalanine hydroxylase enzyme is formed, thus leading to a severe classical PKU. Determination of haplotypes and DNA fingerprint patterns indicates a paternal origin of the de novo mutation.