This fact sheet uses a question-and-answer format to summarize what is known about phenylketonuria (PKU), an inherited metabolic disease that leads to mental retardation and other developmental disabilities if untreated in infancy. Questions and answers address the following topics: what PKU is; how PKU is inherited; the diagnosis of PKU; the accuracy of PKU testing and guidelines for testing; the treatment of PKU; the current recommendation that those affected by PKU remain on a restricted diet indefinitely; the associated risks that pregnant PKU mothers face; and screening for other metabolic disorders associated with mental retardation. (DB)