In order to evaluate the argument whether or not a restricted phenylalanine diet should be maintained for life in patients with phenylketonuria (PKU), 16 patients with early treated PKU but off diet since their 11th birthday were investigated. The evaluation included a detailed neurological examination, IQ, neurophysiological testing and MRI of the brain. Even if IQ and electrophysiological studies were normal or unchanged if compared to results before diet discontinuation, all patients revealed abnormal neurological signs. We conclude that the diet should be continued during adult life, but somewhat higher phenylalanine levels (<10mg/dl;<600 micromol/l) than at younger ages should be allowed.