Twenty patients with PKU or hyperphenylalaninemia at ages 0.1 to 15.6 years (median age 6.2 years) were studied prospectively. In all children the condition had been diagnosed when they were neonates on the basis of an abnormal Guthrie test. To maintain plasma phenylalanine levels between 0.2-0.5 mM, dietary restriction of phenylalanine to 20-80 mg/kg daily (median 40 mg/kg) was necessary in 14 children. In children above 8 years, however, these plasma levels were frequently exceeded. In 6 children plasma phenylalanine levels were higher than normal diet. Height, weight and head circumference were within normal range in all patients at all ages. Determinations of DQ/IQ were done at 2, 4, 6 and 8 years of age and revealed values between 90-120 with a median of 102 in the 14 patients who were tested. Only 1 patient had IQ levels between 75-85 and attended special school. Nine other patients were in grade school performing averagely or above. This study confirms that early treatment and long-term follow-up of patients with PKU yield good results. Unsolved problems include duration of dietary treatment and the management of pregnancy in women with PKU.