The normal range for phenylalanine in preterm infants is 1 to 3 mg/dL, and phenylalanine 3.4 mg/dL in preterm infants is considered excessive phenylalanine. Phenylalanine 3.4mg/dL in preterm infants may be caused by delayed maturation of phenylalanine hydroxylase or phenylketonuria in preterm infants. 1. Delayed maturation of phenylalanine hydroxylase: preterm infants’ organs and systems are immature, and enzymes in the body may not be mature, resulting in delayed maturation of the body’s phenylalanine hydroxylase, which affects the metabolism of phenylalanine, resulting in elevated phenylalanine. With the development of preterm babies, phenylalanine hydroxylase gradually matures, and the condition of the children will improve. 2. Phenylketonuria: premature infants due to congenital chromosomal abnormalities leading to phenylketonuria, the body of the phenylalanine hydroxylase deficiency, phenylalanine can not be metabolized to tyrosine, causing elevated blood phenylalanine. Phenylalanine 3.4mg/dL in preterm infants should go to the hospital in time to improve the examination and give targeted treatment after clarifying the cause of the disease.