Newborn screening and early treatment prevent the most severe manifestations of phenylketonuria (PKU). However, executive functioning deficits, attention deficit disorder, slow processing speed, and visual-motor problems commonly occur. Many adults with this disorder also suffer depression and anxiety. Using advanced electroencephalogram (EEG) and magnetic resonance imaging (MRI) techniques, including novel MR spectroscopy (MRS) we hope to discover why this distinct constellation of deficits occurs in PKU. Adult subjects with PKU will undergo EEG and comprehensive MRI evaluations, including a novel method of MR spectroscopy to determine brain phenylalanine levels. In addition, they will receive neurological and neuropsychological examinations and dietary evaluation.