7 patients with phenylketonuria who developed neurological disability in adolescence or early adult life are described. 4 had been diagnosed by routine neonatal screening and started a low phenylalanine diet in infancy. 3 were diagnosed in early childhood because of developmental delay, and then started dietary treatment. Dietary control deteriorated in later years and was withdrawn in mid to late childhood. The late neurological deterioration cannot be directly ascribed to poor compliance with or cessation of dietary treatment in this small, retrospective study--but other likely causes have been excluded and 2 patients showed a striking clinical improvement when a strict diet was resumed. Serial magnetic resonance images from one of these patients show abnormalities that appeared after cessation of dietary treatment and resolved after diet was resumed. If these findings are confirmed, strict dietary control into adult life would be indicated for at least some patients with phenylketonuria.