Results of psychological testing of patients aged 3-6 years.

Cognitive development of early treated PKU children (132) was investigated by intelligence tests, tests for visual perception, motor and language development. Deviations from test norms occurred at 5 years of age concerning performance IQ, mathematical thinking, and visual perception. With the exception of mathematical thinking, "normal" results were obtained at the age of 6 years. Correlation with levels of plasma Phe revealed a relation between quality of dietary control and performance IQ. Results are discussed in light of methodological aspects and with respect to treatment consequence.