Sixty-five children with pervasive developmental disturbance (autism and atypical childhood psychosis) were screened by standard urinary amino acid detection testing methods. Three of the children showed abnormalities in these screening tests, leading to the diagnosis of phenylketonuria. This was verified by repeated urinary testing and blood phenylalanine determinations. The children with phenylketonuria were treated with low-phenylalanine diets and have shown improvement in functioning and developmental level since treatment. Urinary genetic screening should be a standard test for all children being evaluated for serious developmental disturbances of childhood.