In a retrospective study, 34 treated adolescents with phenylketonuria and their relatives were tested with scale 2 of the Culture Fair Intelligence Test (CFT20) and self-developed questionnaire concerning their development in school. The patients also filled in the Anxiety Questionnaire for Children. With an IQ of 93.6, the patients reached a normal mean intellectual performance but this was significantly below the mean IQ of the general population (100, p < 0.01), the IQ of their mothers (98.2, p < 0.05), their fathers (105.4, p < 0.05) and their siblings (110.3, p < 0.05). The patients' IQs correlated significantly with the IQs of their mothers, the socio-economic status of their families, the quality of dietary control since birth and the serum phenylalanine concentration at the moment of testing. With respect to age at which the patients started school, type of school attended and number of school years which had to be repeated, there were no significant differences between the patients and their siblings. The patients did not display a markedly higher degree of general anxiety, nervousness at examination time or a greater reluctance to attend school.