Newborn screening for phenylketonuria began 35 to 40 years ago in most industrialized countries. Because of this initiative, which resulted in early institution of phenylalanine-restricted diets, there are now many young adults with this disease who have normal or near-normal intellectual function. In North America alone, 200 patients with phenylketonuria enter adulthood every year. Most expert panels recommend following a phenylalanine-restricted "diet for life." However, there are few adult physicians dedicated to continuing care of this group, with the possible exception of maternal phenylketonuria. Up to 10% of adults with classic phenylketonuria, and possibly 50% of those with milder variants, may not need treatment; after adolescence, intelligence does not appear to deteriorate, at least into early adulthood, even if diet therapy is discontinued or not in good control. However, neuropsychological and psychosocial problems develop frequently, needing focused and intensive support by health care providers. New investigative methods and treatment options are on the horizon. There is an urgent need for physicians who will orchestrate the care of adults with phenylketonuria.