Early-treated PKU children were compared to their matched non-PKU sibling controls on Wechsler Intelligence Scale for Children (WISC) and Wide Range Achievement Test (WRAT) results at age 8. Fifty-five PKU children had mean WISC Full Scale IQ score of 100, in comparison to a mean of 107 for their matched sibling controls (p = 0.001). Treatment parameters significantly correlated with sibling-PKU IQ score differences included maximum diagnostic phe level (r = 0.244, p = 0.036) and phe levels at age 6 (r = 0.329, p = 0.007) and at age 8 (r = 0.489, p less than 0.0005). Fifty PKU subjects scored significantly lower than their matched sibling controls on standard scores of the WRAT Reading (102 vs. 107, p = 0.016) and Arithmetic (96 vs, 101, p = 0.006) subtests, and lower, but not significantly so, in Spelling (100 vs. 103, p = 0.145). When the sample was grouped according to diet status at age 8, on-diet PKUs scored at or above the level achieved by their siblings on all three scales of the WISC and all three WRAT subtests, whereas the off-diet group scored from 7 to 13 points below their siblings on all measures. These results suggest that PKU children should restrict phe intake at least through their school years.