Information was available on 293 family members and spouses of seven generations in an Icelandic family with high frequency of cleft of secondary palate and ankyloglossia. The authors have personally investigated 182 individuals in generations IV-VII and have drawn blood from over 100 members for genetic marker studies. The senior author, Dr. Björnsson, has operated on two-thirds of the affected individuals. Twenty-six family members had cleft palate (CP) and, of these, 19 (17 male and two females) had ankyloglossia as well (CP + A). Twenty females and one male had only ankyloglossia (A). All mothers in one of two branches of the family who had sons with CP + A had ankyloglossia themselves. This was not the case in the other branch, in which the mothers of affected sons were themselves unaffected. Fathers affected with CP, CP + A, or high vaulted palate (HVP) never had affected sons. As reported earlier, the condition has been mapped to the q13-q21 region of the X chromosome using restriction fragment length polymorphism (RFLP) techniques (Moore et al, 1987). Our conclusion is that this midline defect is X-linked but varies in the severity of expression.