Thirty-six patients with a confirmed diagnosis of phenylketonuria (PKU) were identified and placed on dietary therapy before reaching 121 days of age. These children are currently between 4 and 6 years old, and have been given the Stanford-Binet Intelligence Scale. Subsequently, their normal siblings of closet age were selected as matched-pair controls and were also tested with the Stanford-Binet. The mean age of the PKU sample when tested was 50.0 months, and 50.9 months for the normal controls. The 36 index patients scored a mean IQ of 94 and their nonaffected siblings obtained a mean IQ of 99. This five-point mean difference was significant at the .02 level, and suggests the presence of a minimal intellectual impairment associated with PKU, even when treatment begins early and is rigorously monitored.
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Dobson et al. (1976) studied this question.