In phenylalanine hydroxylase deficiency detected by screening treatment in early life, both age at start of treatment and phenylalanine control during treatment are the major determinants of eventual psychological status. The influence of phenylalanine control declines with age but executive performance is influenced by hyperphenylalaninaemia at all ages. In a few subjects neurological deterioration has been reported years after relaxing or stopping treatment. MRI changes in brain white matter are present in most subjects no longer on a strict diet. These changes are usually reversible and closely related to phenylalanine status at the time of investigation. Whether or not the changes point to a specific vulnerability of white matter remains uncertain, although MRI changes were particularly prominent in subjects with neurological disability and may be irreversible in such subjects. Policies on treatment have to take account of these findings.