Prenatal diagnosis of tetrahydrobiopterin (BH4) deficiency was undertaken by evaluating the pterin patterns in amniotic fluid and the specific enzyme activities in fetal or extrafetal tissues. This allowed the prenatal diagnosis in 19 pregnancies at risk. In 8 families with a child already affected by dihydropteridine reductase deficiency 4 fetuses were diagnosed as homozygotes and 4 as heterozygotes for the defect. In 11 families with a child affected by 6-pyruvoyl tetrahydropterin synthase deficiency 4 fetuses were homozygous, 4 heterozygous and 3 normal. This study also advanced our knowledge of tetrahydrobiopterin metabolism during fetal development. The key enzymes involved in the biosynthesis of BH4 are expressed early and allow the fetus to be autotrophous for its cofactor requirement. In a twin pregnancy, both fetuses were diagnosed to be heterozygotes for dihydropteridine reductase deficiency and primapterin (7-biopterin) in amniotic fluid was increased. This indicates that pterin-4 alpha-carbinolamine dehydratase activity seems to be differently expressed during fetal life. As a consequence, pterins detected in amniotic fluid are of fetal origin and 6- and 7-substituted pterins can be present in amniotic fluid in higher proportions when compared with other body fluids.