Natural history of phenylketonuria and influence of early treatment.

This study describes the intellectual development of twenty-eight sibling pairs with phenylketonuria in which the index case presented because of retarded development. In twelve pairs (group I) the "second" sibling was born subsequently, and in sixteen (group II) there was a second sibling already in the family at the time of diagnosis of the index case. The intellectual outcome of the second siblings of group II suggests that approximately 1 in 6 or 7 patients with untreated classical phenylketonuria achieves an I.Q. above 70, although with evidence of intellectual impairment. The highly significant difference in intellectual outcome between the early treated second siblings of group I and the late or untreated second siblings of group II demonstrates the beneficial effect of early treatment on the intelligence of affected children.