In trisomy 18, echocardiographic studies of affected neonates and pathological studies of stillbirths and infants have demonstrated a high incidence of cardiac defects. Fetal trisomy 18 can now be detected at 11–14 weeks of gestation, providing the opportunity to examine the incidence of cardiac defects at this gestational age. In 19 fetuses with trisomy 18 pathological examination of heart and great vessels was carried out after termination of pregnancy at 11–14 weeks of gestation. The abnormal karyotype was diagnosed by chorion villus sampling in pregnancies with increased fetal nuchal translucency thickness. All 19 fetuses had cardiac defects, and the commonest were ventricular septal defects in 16 (84%) and valvular abnormalities in 16 (84%). In 14 of the 16 cases with valvular abnormalities more than one valve was affected. The great vessels were available for examination in 18 of the 19 cases, and in 10 there was a hypoplastic aortic isthmus or pulmonary trunk. In 6 (32%) cases there was persistence of the left superior vena cava. In 1st-trimester fetuses with trisomy 18, the frequency of perimembranous ventricular septal defects and valvular abnormalities is similar to that in affected neonates. Haemodynamic changes due to the valvular abnormalities, especially imperforate valves, and hypoplasia of the great vessels may be the underlying mechanisms for the increased nuchal translucency of trisomic fetuses. Persistence of the left superior vena cava may result from venous congestion of head and n