The mean I.Q. of 27 PKU patients at the mean age of 3 years 10 months treated before 3 weeks of age was comparable to that of their unaffected siblings. The mean I.Q., of 12 patients at the mean age of 5 years 6 months treated between 3 and 6 weeks of age fell significantly below the mean I.Q. of their unaffected siblings. Seventeen patients treated after 8 months of age did not differ in their mean I.Q. at the end of treatment from 11 untreated patients, despite the observation that more than half of the late-treated patients achieved significant gains during the course of their treatment. Patients off dietary therapy did not appear to deteriorate over a follow-up duration of 2 to 3½ years. Eleven were followed for 6 years without evidence of deterioration. Thirteen atypical PKU's showed normal intellectual development. An unexplained predominance of males was observed in our group of atypical PKU patients.

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