Introduction: This report describes the system of neonatal screening for phenylketonuria (PKU) that has been developed in Turkey and the results of screening. Methods: A total of 576 122 newborns born in maternity hospitals in metropolitan districts of the cities were screened using the Guthrie bacterial inhibition test. Classification of PKU or non-PKU hyperphenylalaninemia was determined by the confirmatory serum phenylalanine level and/or subsequent protein loading test. Results: One hundred and forty-two cases with persistent hyperphenylalaninemia were detected (1:4500), In 96 of these cases confirmatory tests were found to be compatible with PKU (1:6000) and the remaining 46 cases had either mild or moderate hyperphenylalaninemia. In one patient, the hyperphenylalaninemia was secondary to a pterin defect (dihydropteridine reductase deficiency). The ratio of PKU/non-PKU hyperphenylalaninemia was 2.0. Discussion: The major problems encountered in the screening program and in management of the detected cases were unsatisfactory sample collection, early discharge from maternity hospitals, difficulties in reaching some of the detected cases, noncompliance with dietary therapy due to illiterate parents or in families without social insurance. The health system in Turkey seems to require a second blood sampling for PKU screening obtainable in Maternal and Child Health care units in the cities and by midwives in rural areas, In Turkey, with an exceptionally high frequency of PKU, prenatal diagnosis for PKU should be combined with the neonatal screening program.