Seventy-two adults with phenylketonuria were evaluated to investigate the genotypic relationship to phenotype. Patient data were collected by chart review and medical follow-up as well as current psychological evaluation. Nineteen diagnosed neonatally had remained on a phenylalanine-restricted diet all their lives, whereas 34 who were also diagnosed on newborn screening had discontinued dietary restriction during childhood. Nineteen others who were born prior to newborn screening were diagnosed later than the newborn period on clinical grounds but have remained on dietary restriction. Comparison between intellectual ability, academic achievement, and mental illness was made with degree of diet control as defined by range of blood phenylalanine levels over time. Diet discontinuation in childhood did not significantly lower IQ per se but appeared to diminish academic achievement. The lowest IQ scores were associated with poor dietary restriction of phenylalanine in the diet during childhood. While there appears to be a strong genotypic relationship to phenotypic metabolic parameters in phenylketonuria, there does not seem to be a similar relationship to intellectual ability in adults. Mutation R408W was not strongly related to the occurrence of mental illness in this sample. We conclude that dietary restriction of phenylalanine neonatally and good control contributed to normal intellectual development. Continuation of dietary treatment into adulthood appeared to improve academic achievement in patients with severe phenylalanine hydroxylase mutations.