A series of 467 cases of congenital heart disease detected in prenatal life were analyzed to identify the forms of cardiac malformation associated with karyotypic defects and to calculate the incidence of chromosomal abnormalities associated with such malformations. Of these, 77 were proved to have chromosomal anomalies although not all were karyotyped. The results were analyzed in two ways. First, the whole series of 77 cases was examined in order to describe the form of congenital heart disease found in association as, although many cases were forms of heart disease known to be associated with chromosomal defects, several were unexpected. In many cases which proved positive, there were other abnormal findings on ultrasound, further suggesting a high likelihood of a chromosomal defect. Some cardiac defects, however, such as atrial isomerism or transposition of the great arteries, were not associated with trisomies. The second part of the study examined in detail the records of 124 cases of congenital heart disease seen in 1989, as these were more completely documented than the previous cases. No chromosomal anomaly was therefore thought to be missed. Of this group, 20 (16%) proved positive. This is a higher rate than would be expected in an unselected population of live births. The difference between prenatal and postnatal life can be accounted for by the increased rate of spontaneous fetal loss in those with chromosomal defects. We conclude that, because of the high rate of chromosomal anomaly, all continuing pregnancies where congenital heart disease has been found in the fetus should be karyotyped unless specific types of heart defect which are rarely associated are confidently defined.