Twenty-two French patients with early treated phenylketonuria (PKU) off diet (no reduced phenylalanine, Phe) since their 5th birthday, 23 German patients on diet (reduced Phe), and 21 healthy control subjects from childhood to adulthood matched for age, sex, and IQ were investigated for visuomotor reaction time, sustained attention, and visual stimulus scanning. Determinations were made whether1) the three groups showed different developmental trends in their reaction times,2)the threshold of a Phe blood level of 360 μmol/L formulated in recent recommendations can be regarded as safe,3)test performances are related to the quality of dietary control in the same way for all age groups, and4) long-term elevated Phe levels result in aggravating effects of increasing differences between patients on and off diet and healthy controls. Results revealed that developmental trends were similar in all treatment groups. Only children with a mean Phe level below 360 μmol/L performed as well as control subjects. Differences between treatment groups decreased in adulthood, and no aggravating effect could be observed. Mean performance of patients with mild PKU off diet was in the same range as performance of patients with classical or mild PKU on diet, calling into question the benefit of treating these patients. It is concluded that it is preferably safer to maintain Phe blood levels below 360 μmol/L at least during the first 10 y of life.Abbreviations: DPE,dot pattern exercise;FMSE,finger motor speed exercise;LPE,letter pattern exercise;Phe,phenylalanine;PKU,phenylketonuria;SDS,standard deviation score;SDST,standard deviation of series time;ST,series time;SVAT,Sonneville visual attention task;WAIS-R,Wechsler Adult Intelligence Scale-Revised;WISC-R,Wechsler Intelligence Scale for Children-Revised