The various conditions which can lead to elevated blood phenylalanine (PHE) levels must be differentiated promptly in the neonatal period so that the correct treatment can be implemented as soon as possible. In order to exclude the rare tetrahydrobiopterin (BH4) deficiency, it is advisable to perform a BH4 loading test and to determine the renal excretion of pterins, as well as the dihydropteridine reductase activity in erythrocytes. The practical consequence of differentiating the various types of PHE hydroxylase deficiency is that with both phenylketonuria (PKU; PHE greater than 20 mg/dl) and hyperphenylalaninemia (HPA) with PHE levels above 15 mg/dl a diet restricted in PHE is initiated, whereas HPA infants with PHE levels below 8 mg/dl are fed normally. In the case of PHE concentrations between 8 and 15 mg/dl standardized protein intake can be used to decide whether a diet restricted in PHE or only a restriction of protein intake has to be instituted. Optionally a protein challenge can be performed at the age of six months in order to evaluate the individual PHE tolerance. Psychometric investigations of PKU patients after diet discontinuation at different ages, as well as animal studies are in favour of a diet-for-life. The diet in PKU patients is known to lead to some side effects such as bony changes and amino acid imbalance, as well as deficiency of selenium and carnitine. Finally, great efforts have to be made in order to avoid the increasing danger of PHE embryofetopathy in the offspring of PKU mothers (maternal PKU).