Pathological examination of the heart and great arteries was performed in 112 chromosomally abnormal fetuses after surgical termination of pregnancy at 11-16 weeks of gestation. The chromosomal abnormalities were diagnosed by chorion villus sampling which was carried out because screening of the pregnancies by a combination of maternal age and fetal nuchal translucency thickness at 10-14 weeks of gestation identified them as being at increased risk. The group consisted of 60 fetuses with trisomy 21, 29 with trisomy 18, 17 with trisomy 13 and 6 with Ullrich-Turner syndrome. The most common cardiac lesion seen in trisomy 21 fetuses was an atrioventricular or ventricular septal defect. Trisomy 18 was associated with ventricular septal defects and/or polyvalvular abnormalities. In trisomy 13, there were atrioventricular or ventricular septal defects, valvular abnormalities, and either narrowing of the isthmus or truncus arteriosus. Ullrich-Turner syndrome was associated with severe narrowing of the whole aortic arch. In all four groups of chromosomally abnormal fetuses, the aortic isthmus was significantly narrower than in normal fetuses and the degree of narrowing was significantly greater in fetuses with high nuchal translucency thickness. It is postulated that narrowing of the aortic isthmus may be the basis of increased nuchal translucency thickness in all four chromosomal abnormalities.