In order to examine the association between treatment parameters and intellectual outcome, a total of 58 adolescents with phenylketonuria (PKU) treated at the PKU center of the Department of Pediatrics at the University of Prague were tested at the age of 15 years with the Prague Child Wechsler Test. Patients were classified into three groups with different levels of dietary control that remained relatively stable over time at mean phenylalanine levels of 360, 540 and 680 mumol/l. The mean level of phenylalanine was highly correlated with IQ and school education. Patients with mean phenylalanine levels of < or = 360 mumol/l had near-normal level. Initial experience with eight cases of maternal PKU underlines the necessity for a strict treatment regimen, which must be started pre-conception in order to prevent the fetus from developmental damage.