A screening program for phenylketonuria in newborn infants that is being carried out at one of the Metropolitan Health Services at Santiago, Chile for the last 17 months, using the Guthrie test, is described. During this period 15,214 blood samples have been analyzed, which represented 94.4% coverage for beneficiary newborn infants. Two cases of transient hyperphenylalaninemia, one patient with benign hyperphenylalaninemia, and one infant with classical phenylketonuria have been thus identified. In this last child nutritional management was started at 13 days of life. Screening programs for early detection of phenylketonuria in Chile seem convenient, feasible and reliable.