Monozygotic twins with concordance for Down's syndrome and congenital heart disease.

Cytogenetic, clinical, and other findings are presented on a pair of same-sex, monochorial MZ twins concordant for both trisomy 21 and congenital heart disease. The literature on Down's syndrome in twins and on congenital heart disease in twins is reviewed and the exceptional occurrence of concordant congenital heart disease in MZ twins is stressed.