Differences in cord length between weight-concordant pairs were studied in 43 monochorionic, 59 same-sex dichorionic and 42 different-sex dichorionic sets of twins and comparisons were made between groups. Of obvious and curious note was the narrow range of cord length differences between twins in monochorionic pairs (range, 0-5 cm; mean, 2.1 +/- 1.6 cm). Same-sex and different-sex dichorionic groups had significantly wider ranges of cord length differences than the monochorionic group (range, 0-34 cm; means, 7.7 +/- 6.4 and 11.0 +/- 6.1 cm, respectively). In the same-sex dichorionic group no pair whose cord lengths differ by > 5 cm has yet been found to be identical. Discordant monochorionic twins had greater differences in cord length. All other factors being equal, the remarkably similar cord lengths of concordant identical twins strongly supports the hypothesis that a genetic component influences cord length.