Thirty patients with early and continuously treated phenylketonuria (PKU) between 8 and 20 years of age were compared with 30 controls, matched individually for age, sex, and educational level of both parents, on behaviour rating scales for parents and teachers as well as a school achievement scale. PKU patients, as a group, demonstrated more problems in task-oriented behaviour and average academic performance than did matched controls. Interestingly, whereas male PKU patients were rated significantly lower on introversion by their teachers, female patients were rated significantly higher on introversion and lower on extraversion than matched controls. This sex difference was also reflected in the relationship between measures of dietary control and the behaviour clusters, suggesting that male and female patients respond differently to elevated Phe levels or the stress associated with PKU. The teacher rating on average academic performance of the PKU patients was associated with recent level of dietary control, which suggests that it might be improved by more strict adherence to the diet. In addition, academic performance correlated negatively with the behaviour cluster negative task orientation. Further studies are recommended to obtain a more complete evaluation of this relationship and to replicate the current findings on larger samples.