Amniotic fluid endothelin-1 (ET-1) levels were measured in 38 euploid and in 15 aneuploid pregnancies in the 17th gestational week. Varying distribution of the peptide levels was found in the two groups, with higher values in the pathological cases. Should this finding be confirmed in maternal blood, ET-1 could represent a further analyte to be used in prenatal screening for aneuploidy.