The prenatal diagnosis of two subsequent pregnancies of the mother of a patient homozygous for a mutation in the L-2-hydroxyglutarate dehydrogenase gene is described. In the first pregnancy, measurement of L-2-hydroxyglutaric acid in amniotic fluid revealed an affected fetus. This pregnancy was terminated. In the prenatal diagnosis of the second pregnancy, mutational analysis was also included and proved to be of pivotal importance. Despite mildly increased levels of L-2-hydroxyglutaric acid in the amniotic fluid, the fetus was heterozygous for this mutation thus excluding L-2-HGA.