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. Over the years a number of studies have examined behaviour and school achievement in patients with early treated phenylketonuria (PKU; McKusick 261600). In general, these studies have found that despite early treatment with a phenylalanine (Phe)-restricted diet, PKU patients demonstrate more behavioural and school problems than do healthy controls. The behaviour problems include both internalizing symptoms (e.g. solitary, unresponsive, anxious, depressed mood: Pietz et al 1997; Smith et al 1988; Weglage et al 1992) and externalizing symptoms (e.g. hyperactive, talkative, impulsive, restless: Hendrikx et al 1994; Kalverboer et al 1994; Realmuto et al 1986; Smith et al 1988), but not antisocial or socially negative symptoms (e.g. lying, teasing, disobedience: Kalverboer et al 1994; Pietz et al 1997; Smith et al 1988). With respect to school achievement, studies have shown that patients with early treated PKU more often repeat classes or need special tutoring (Berry et al 1979; Brunner et al 1983; Koch et al 1987; Rey et al 1996; Verkerk 1995), have to work harder than healthy controls to achieve the same results (Weglage et al 1993), or have specific deficits in arithmetic achievement scores (Azen et al 1991; Berry et al 1979; Fishler et al 1987; Koch et al 1987; Weglage et al 1993). Nevertheless, many questions regarding the behavioural and school problems of patients with early treated PKU remain unanswered. For instance, the relationship between behavioural and school problems on the one hand and levels of dietary control on the other is still relatively unclear. The few studies that examined this relationship, have focused primarily on children in primary school (Azen et al 1991; Koch et al 1987; Smith et al 1988). Furthermore, although several psychological studies have shown that the pattern of behavioural problems varies by sex (see Prior et al 1999a for a discussion), so far very few studies have examined this issue in PKU patients and results are contradictory (Kalverboer et al 1994; Pietz et al 1997; Smith et al 1988; Weglage et al 1992). In addition, so far no study has actually examined whether there is a relationship between the behavioural problems and school difficulties of PKU patients, even though this relationship has been well documented in the psychological literature (Prior et al 1999b; Richards et al 1995). The aim of the present study is therefore to examine these issues in patients with early and continuously treated PKU over a wide age range and in relation to dietary control. More specifically, school achievement as well as social and task-oriented behaviour (at home