A metabolic disorder where there is a deficiency of the enzyme phenylalanine hydroxylase which leads to a harmful buildup of the phenylalanine in the body. Normally the phenylalanine is converted into tyrosine. The severity of the symptoms can range from severe enough to cause mental retardation to mild enough not to require treatment. Severity is determined by the level of impairment of enzyme activity of phenylalanine hydroxylase. See also Phenylketonuria