Individuals with late-treated phenylketonuria (PKU) not detected by newborn screening but who followed dietary treatment for at least 12 months before 7 years of age have intelligence quotient (IQ) scores that range from severe impairment to the low-normal range. Among adults with late-treated PKU in California, 85% of those who were born from 1961 to 1978 had IQ scores of 70 or above. Longitudinal studies with repeated cognitive assessments often show average changes in cognitive test scores as high as 20–45 points. Although the severe cognitive impairment associated with untreated PKU can in many cases be partially reversed with dietary treatment, prompt initiation of treatment following newborn metabolic screening is essential for optimal development and the prevention of disability.