Forty years after Horst Bickel first treated a patient suffering from phenylketonuria (PKU) our aim is to assess the current treatment of Austrian patients. A total of 70 children -60 with PKU and 10 with hyperphenylalaninaemia (HPA)-aged 6-16 years were investigated in terms of somatic and intellectual parameters. Their development is normal (PKU: mean IQ = 95.40; HPA: mean IQ = 101.85) owing to strict dietary control, above all during their first 6 years of life. A comparison of the IQ data of 17 PKU children and their healthy siblings at the age 6 years showed significant correlations in verbal, performance and IQ measurements. Austrian PKU patients do achieve normal IQ values but these measurements fail to guarantee the quality of dietary control. IQ is influenced by a number of variables: genetic, social environment, education and furtherance, motivation for performance, etc., and, in PKU patients, dietary control. The only currently known way of maintaining and improving dietary compliance in PKU patients and their families remains good informative counselling about the disease and psychosocial support.