Maternal hyperphenylalanemia during pregnancy may induce a severe embryopathy characterized by microcephaly, mental retardation, facial dysmorphy and congenital heart defects. Four subjects, two pairs of sibs, with maternal hyperphenylalaninemia syndrome were included in this study and their neuropsychological performances were assessed. Maternal levels of hyperphenylalaninemia were similar in both mothers, one of them had not been diagnosed with the condition until her two children were examined at the ages of 10 and 6 years. A severe cognitive deficit was detected in all 4 subjects, with a typical profile of impaired perceptive abilities, behavioural disturbances, motor difficulties and poor familiar integration.