The distribution of congenital heart lesions was studied in 238 families with at least 2 affected members. A statistical analysis was performed. Concordant lesions were found in 48% of the affected first degree relatives and in 28% of the affected second and third degree relatives. The concordance rate is highly significant for all lesions studied in the first degree relatives, with the exception of ventricular septal defect (VSD). Among the discordant pairs of lesions, some occur significantly more often than expected (tetralogy of Fallot associated with VSD, pulmonary stenosis, and transposition of the great arteries); others, such as the association between VSD and pulmonary stenosis, are significantly less common than would be expected on a random hypothesis. An explanation is proposed suggesting that malformations anatomically dissimilar but resulting from the same heart segment disorder may have some common genes, and that interaction between genes may be responsible for "antagonism" between 2 defects. The embryologic segmental approach to congenital heart disease is reinforced by this genetic study.