In the past, it was believed that nearly all patients afflicted with this disorder were mentally retarded. In a survey covering all published cases, Jervis 1 found that only three out of 330 phenylketonuric children (0.9%) had an intelligence quotient above 70. In 1960, Knox2 reported that there were 20 known untreated patients with phenylketonuria having "high grade" intelligence. Of these, eight patients had IQs between 60 and 69; six patients had IQs between 70 and 79; three patients had IQs between 80 and 89; and three patients had IQs over 90. More recently, Knox3 has expressed the view that no single factor can account for the relatively mild intellectual defect in these "atypical" phenylketonuric children. He believes that some of these have a biochemical defect which is often as severe as in the usual patient with phenylketonuria. But there appears to be a second group of patients with lower plasma phenylalanine levels; a few also have mildly affected relatives.The widespread screening for phenyl¬ ketonuria among newborn infants has led to the recognition that not all instances of hyperphenylalanemia are caused by