SummaryAmniotic fluid samples from pregnancies complicated by foetal methylmalonic aciduria and from metabolically normal pregnancies were obtained at 16–18 weeks of gestation and analysed for total, free and acylcarnitine and individual carnitine esters. The amniotic fluid concentrations of total acylcarnitine and propionylcarnitine were higher in pregnancies with methylmalonic aciduria than in normal pregnancies. The predominant carnitine ester was propionylcarnitine in the methylmalonic aciduria group and acetylcarnitine in the normal group. These findings suggest that in methylmalonic aciduria, abnormalities of carnitine metabolism already occur early in gestation. The amount of propionylcarnitine in amniotic fluid may be useful as an additional indicator of foetal methylmalonic aciduria