Until recently, the diagnosis of Smith-Lemli-Opitz syndrome (SLOS), an autosomal recessive malformation/mental retardation syndrome, was made on the basis of clinical criteria alone. As a result, prenatal diagnosis has been possible only if sonography disclosed distinct fetal malformations in a subsequent pregnancy. However, the recent description of increased levels of 7-dehydrocholesterol (cholesta-5,7-dien-3 beta-ol) in patients with SLOS, most likely caused by a deficiency of 3 beta-hydroxysteroid-delta 7-reductase, has provided an apparently reliable biochemical marker for diagnosis of SLOS. To determine if this abnormality of sterol metabolism has utility for prenatal diagnosis of SLOS, we measured the levels of neutral sterols in stored amniotic fluid samples from two SLOS pregnancies. In both cases, the diagnosis of SLOS was made in the neonatal period by clinical criteria and the finding of markedly increased levels of 7-dehydrocholesterol in plasma. Quantitative analysis by gas chromatography of sterols extracted from the amniotic fluid of both pregnancies revealed similar, markedly increased levels of 7-dehydrocholesterol and its precursor, lathosterol (cholest-7-en-3 beta-ol), both of which were undetectable in reference amniotic fluids. These findings suggest that abnormalities of cholesterol biosynthesis in SLOS may be sufficiently expressed in fetal life to permit prenatal diagnosis of this disorder by measurement of 7-dehydrocholesterol in amniotic fluid.