Newborn screening of inborn errors of metabolism leads to the early diagnosis of autosomal recessive phenylketonuria and normál development of affected homozygotes due to dietary therapy. Patients with classic symptomatology of phenylketonuria are nowadays fořty years old and older. The treated patients show normál mental development, are married and háve children. The children of female patients develop normally if the women return to the strong low phenylalanine diet at least 3 months before the conception, and their sérum level of phenylalanine is regularly controlled in pregnancy. The čase of a boy with severe congenital microcephaly, etiology of which was recognized only at the age of 10 months, shows that teratogenic phenylketonuria-related embryopathy should be kept in mind, especially because it is fully preventable.

        Key words: congenital microcephaly, mental retardation, teratogenic embryopathy, phenylketonuria disorder of brain in heterozygote