Congenital cardiovascular malformations are a major cause of infant mortality in the United States. In 1985, six defects -- hypoplastic left heart, tetralogy of Fallot, truncus arteriosus, transposition of the great arteries, endocardial cushion defect, and ventricular septal defect -- were among the fifteen most frequent causes of premature death due to congenital malformations, accounting for 109,063 years of life lost. This loss of productive life continues in spite of major advances in medical and surgical treatment of congenital cardiovascular malformations. Even as successful treatment modalities are developed, the survival to reproductive years without improved understanding of genetic risks produces additional unanswered questions.