Twenty-seven children with phenylketonuria who had undergone dietary restriction of phenylalanine since infancy were administered a battery of neuropsychologic tests in childhood. Children without PKU were also assessed. Discriminant function analysis of the neuropsychologic measures resulted in correct diagnostic classification for 94% of the total sample. Measured intelligence, school achievement, concept formation, and tactile-motor problem solving were the most powerful discriminators. In general, motor speed and coordination were not significantly different in patients compared with nonpatients. Serum phenylalanine concentration on the day of neuropsychologic testing was negatively correlated with performance. Correlation coefficients between infant serum phenylalanine concentrations and later neuropsychologic performance did not reach statistical significance. We suggest that concurrent serum phenylalanine concentrations affect neuropsychologic performance and that therefore the practice of terminating dietary restriction requires further scrutiny.