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Of the 216 children with phenylketonuria (PKU) who were initially enrolled in the Collaborative Study of Children Treated for Phenylketonuria, 203 were placed on dietary therapy between 3 and 92 days of age. Of these, 111 are now at least 4 years of age and constituted the sample for the present analysis. Their mean IQ on the Stanford Binet Intelligence Scale was 93 (1972 norms). The children assigned to two treatment groups based on "moderate" and "low" serum phenylalanine levels were comparable on their IQs at age 4, although many of the children could not be maintained in the specified categories. Females scored a significantly higher mean IQ than males (97 vs. 90). Those children for whom dietary treatment was initiated during the first month of life scored a mean IQ of 95, compared with 85 for those initially treated from 31 to 65 days. However, the interpretation of dietary inception data may have been contaminated by familial and psychosocial factors. The PKU Collaborative Study is still in progress in 15 clinics located in 11 states.

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