Cranial magnetic resonance imaging (MRI) was performed in nine treated adolescents with hyperphenylalaninaemia (HPA) in order to analyse possible changes in myelination. Three patients suffered from type I HPA, four from type II and two from type III (persistent HPA). Images were obtained with a 1.5 T unit using spin-echo-sequences. In all patients with type I or type II HPA, abnormal findings in the cerebral white matter were demonstrated including band-like and/or confluent patchy areas of high signal intensity predominantly in the peritrigonal region, with anterior and posterior periventricular extension and/or involvement of the subcortical white matter. The extent of MRI changes did not correlate with the initiation, duration or quality of dietary treatment. There was also no consistent relationship between electrophysiological changes and white matter abnormalities on MRI. Our findings suggest a disturbance of myelination in patients with treated HPA. These results correspond well with earlier neuropathological and biochemical studies in untreated patients.