Acylcarnitines were assayed in amniotic fluid by isotope dilution tandem mass spectrometry. Control values for propionylcarnitine, C4 and C5 acylcarnitines were established. Propionylcarnitine was elevated by a factor of 5 relative to controls in the amniotic fluid of three pregnancies affected with propionic acidaemia. This may provide a convenient new method for prenatal diagnosis of propionic acidaemia. One pregnancy at risk for propionic acidaemia but unaffected according to methylcitrate analysis had an intermediate level of propionylcarnitine, indicating the need for further studies to determine the range of concentration in probable heterozygotes.