Screening for phenylketonuria (PKU) by measurement of phenylalanine level on a dried-blood spot specimen is recommended for all newborns prior to discharge from the nursery. Infants who are tested before 24 hours of age should receive a repeat screening test by 2 weeks of age. There is insufficient evidence to recommend for or against routine prenatal screening for maternal PKU, but recommendations against such screening may be made on other grounds.