Phenylketonuria (PKU) is an autosomal recessive inborn error of phenylalanine (phe) metabolism caused by a deficiency in the enzyme phenylalanine hydroxylase that converts phe into tyrosine. If left untreated, PKU results in increased phe concentrations in the blood and in the brain, which cause severe intellectual disability, epilepsy, and behavioral problems. These disorders can be prevented if a diet low in phe is introduced. This report focuses on a preterm newborn (gestational age 29 wk) with adequate weight (1290 g) and no family history of PKU. His parents had not received metabolic neonatal screening. A blood sample at 16 d of age and a weight of 1430 g showed phe 420 μmol/L, compatible with mild PKU. Mixed feeding was initiated with a formula free of phe (X-Phe), and breastfeeding was fortified with a contribution of 3.5 g/kg daily (2.5 g X-Phe and 1 g of high-value biological proteins). The next measurements of amino acid levels in the blood and urine were normal, and the progenitors study for PKU was negative. Normal feeding was reintroduced with normal neurologic and metabolic later evolution. The disorders of the metabolism of phe, in most cases, are due to a genetic condition. However, there are infrequent cases of transient hyperphenylalaninemia secondary to delayed maturation of the hydroxylation enzyme system. They are especially significant in premature infants. Although these forms have not been shown to cause sequelae, in view of high levels of phe in the blood, phe consumption must be restricted.