Recent results of research appear to show that over the past few years, clearly more restrictive dietary recommendations have become accepted in the treatment of phenylketonuria (PKU). A lifelong diet low in phenylalanine is the goal, since poor dietary discipline can lead to an appreciable decrease in the level of intelligence and, in rare cases, to severe neurological deficits. Furthermore, neuropsychological symptoms as sequelae of current high serum phenylalanine levels also occur. In the present study, the question as to the development of intelligence and compliance with diet was investigated. The results underscore the need for continuing adherence to a diet on the one hand, and for continuous psychosocial support on the other.