In this study, we correlated cardiac malformations in the various types of conjoined twins with the proposed site of embryologic union and evaluated the possibility of survival after surgical separation. We conclude that surgical separation is only very rarely successful in twins with united hearts, whereas survival in twins with separate hearts is more dependent on the malformations of other viscera. This report deals primarily with conjoined twins in whom anomalies of the heart are the major factor in deciding whether surgical separation is feasible.