Coarctation of the aorta is a serious heart defect that can be successfully treated if identified early. Actual narrowing of the aortic isthmus is very difficult if not impossible to demonstrate on prenatal sonography; however, the indirect sign of discrepant ventricular sizes (right ventricle [RV] greater than left ventricle [LV]) is potentially useful to identify fetuses at risk for having coarctation of the aorta. We report on nine fetuses, 18 to 38 weeks gestation, in which the left ventricle was smaller than the right ventricle. After birth, four of the nine had coarctation of the aorta. One infant with Down's syndrome had a patent ductus arteriosus as well as foramen ovale. Another infant had a small left ventricle and parachute mitral valve but no coarctation. The last three infants had a normal cardiac workup at birth. We conclude that a small left ventricle compared to the right ventricle on prenatal sonography can be a sign of congenital heart disease and that one of the defects that can give this appearance is aortic coarctation. Careful neonatal follow-up is warranted.