The age at which children suffering from classical phenylketonuria can safely discontinue their dietary therapy has been constantly disputed over the past decades. Recently, most phenylketonuria centers have begun to recommend a life-long diet, especially for female patients. Male patients are also advised to continue their diet until at least well into adult age. As a result of this new outlook in therapy management, we reviewed the existing literature and summarized all relevant long-term follow-up data of children who discontinued their debts at an early age, focusing on intellectual and neurological performance. The abilities of these children are compared during dietary treatment and again several years after diet discontinuation. Results show clearly that children maintaining their diets into their teens have fewer deficits than do those terminating their diets before 10 years of age. It seems essential to initiate diet early, and to keep blood phenylalanine levels < 600 mumol/L and well controlled to at least age 10 to ensure satisfactory long-term development of the patient. Furthermore, it seems highly justified to maintain a life-long diet which can be liberalized, but not completely discontinued in adulthood.