DIAGNOSTIC PROCEDURES for testing newborn siblings in families known to have phenylketonuria were outlined, following recommendations of a number of physicians experienced in management of this disease.1,2Subsequent siblings carry a 25% risk of also having phenylketonuria. Testing of infants born into affected families offers the opportunity not only of detecting the disorder very early in life for the purpose of treatment, but also of determining the reliability at a given age of tests which have been proposed for this purpose.3 We have had the opportunity during the past year to test blood and urine specimens from a number of infants born into families with phenylketonuria. Arrangements were made with the attending physician prior to delivery, and materials for collection of blood and urine specimens were supplied. The usual routine of the obstetrical unit and hospital nursery was observed with regard to standard fluids, breast, or bottle feeding.