A sensitive and reliable method for trace analysis of methylmalonic acid in amniotic fluid and urine is described using deuterated methyhnalonic acid as the internal standard and capillary gas chromatography/mass fragmentography. The application of the method for the prenatal diagnosis of methylmalonic acidemia is demonstrated in three pregnancies at risk. In two pregnancies the fetuses were affected by methylmalonyl-CoA-mutase deficiency. Correspondingly, the excretion of methylmalonic acid in the maternal urine was elevated as early as at the 12/13th week of gestation, reaching its highest level shortly before abortion at the 19/20th week: 157 and 173gmol/24h (excretion in normal pregnancies: 39+ 8gmol/24h, n = 8). In addition, the concentration ofmethytmalonic acid in amniotic fluid at the 16th week (13.4 and 33.8gmol/1, normal range 0.31 + 0.10gmol/1, n = 8) strongly suggested that the fetuses were affected. In the third pregnancy no increase of the methylmalonic acid excretion in maternal urine at 11-17 weeks of gestation could be found (42+ 10gmol/24h, n = 5). The cultured amniotic cells of this fetus showed normal enzyme activity. Nevertheless abortion was initiated without further biochemical investigation because of an elevated a~-fetoprotein value in the ananiotic fluid. The fetus was anencephalic. The data suggest that it is possible to make a reliable prenatal diagnosis of methylmalonic acidemia even in those cases where cultured amniotic cells are not available.