Phenylketonuria can now be detected during the first few days of life by two reliable mass screening techniques; and its major consequence, severe mental retardation, can be prevented by the early institution of a low phenylalanine diet. Case finding, based on determination of phenylalanine serum levels in newborns before discharge from the hospital, appears to yield an acceptable number of new cases without excessive numbers of false positive or false negative tests at the 4 mg per 100 ml reporting level. Feeding history does not appear to be a major factor in influencing test results. In addition to finding cases of phenylketonuria, newborn blood screening has called attention to another group of infants with hyperphenylalaninemia of other causes. The differential diagnosis in such cases is important because the restrictive diet necessary for patients with phenylketonuria might be harmful to others. Such factors as enzymatic immaturity, heterozygote carriers, maternal enzymatic capacities and other amino-acidemic states must be ruled out by thorough examination. Careful observation, investigation and reporting of experience with these patients will help to eliminate some of the present deficiencies in the knowledge of normal and abnormal amino acid metabolism.