Congenital malformations have been investigated in a consecutive series of 445 monozygotic twin pairs coming from two twin surveys numbering 1,424 pairs. Concordant malformations were found in 6 pairs, discordant in 20 pairs. The significance of these findings is briefly discussed. The methodology of the prospective twin studies is described. The diagnosis of zygosity is based on sex, structure of placental membranes, blood groups and enzyme systems of the blood and placenta.