This study investigated the relationship between school performance, cognitive functions, and dietary control in a group of 26 early and continuously treated phenylketonuric patients, in comparison with 21 sex- and age-matched control subjects. The cognitive functions study included intelligence measurement, visual and auditory memory and auditory verbal learning abilities, attention, visuospatial, fine motor, language, and executive functions. Participants were asked about school performance. The indexes of dietary control for the first 6 years of life and for the 6 months before the study were calculated. The intelligence score was significantly lower in phenylketonuric patients (P < 0.0001). The percentage of patients with attention problems (P = 0.02), fine motor (P = 0.001) and executive dysfunctions (P = 0.013) was significantly higher than that for control subjects. Patients had more school problems than controls (P = 0.028). Intelligence score was also significantly lower in these patients (P = 0.046). The index of dietary control for the last 6 months was significantly higher than the index for the first 6 years of life, but only in the patients with school problems (P = 0.033). In conclusion, phenylketonuric patients presented more school problems than control subjects, probably related to the disturbed cognitive functions observed. The index of dietary control for the last 6 months yielded a close relationship with school performance.