To investigate the correlation between the abnormalities of magnetic resonance imaging (MRI) of the brain and blood phenylalanine (Phe) levels in phenylketonuria (PKU) and hyperphenylalaninemia (HPA), we reviewed MRIs from 16 patients with early treated PKU and HPA. Their ages ranged from 4-24 years and were found by mass screening and treated from early infancy, and 5 patients with late detected PKU who were aged 24-33 years. The former patients had no remarkable neurological signs or symptoms. One patient of the latter had severe mental retardation and 3 patients had mild to border mental retardation. Axial T1-weighted and T2-weighted spin echo sequences, fluid attenuated inversion recovery MR sequences (FLAIR) through the brain were performed. The scans were graded according to the extent of increased signal intensity of white matter on T2-weighted and FLAIR sequences. To investigate the influence of plasma Phe levels, three approaches were used. Firstly an average of all yearly serial blood Phe concentration was calculated for each patient, then Phe was determined for a period of 6 months and 12 months prior to MRI, and also for their lifetime up to their age at the time this study began. These average blood Phe levels were classified into four categories: group A:Phe level below 5 mg/dl, group B:5-8 mg/dl, group C:9-12 mg/dl, group D:above 12 mg/dl. MRI findings were not significant in group A. Remarkable high signals of white matter were obtained in group C and D, except for one patient in group D whose MRI finding was normal. MRI findings correlated to long-term dietary control stronger than those of 6 months prior to MRI. The clinical significance of MRI abnormalities is still unclear, and further study is required to clarify the relationship of the MRI findings and clinical conditions.