The incidence of cleft lip with or without cleft palate was studied in relatives of Hungarian index patients and found to be 4.86% in sibs, 0.75% in uncles/aunts and 0.31% in first cousins. Relating these figures to the incidence in the general population, first degree relatives have 47 times, second degree relatives 7 times andthird degree relatives 3 times the population incidence of this malformation. The incidence in parents was 1.92%. The heritability estimates were 78% (92% from sibs), 82 and 84% from first, second, and third degree relatives as calculated by the method of Falconer, modified by us. In the present series affected mothers had babies with more severe types of this malformation than had affected fathers. The frequency of cleft lip with or without cleft palate was 6.05% in sibs of patients with cleft lip and palate, but only 3.47% in sibs of patients with isolated cleft lip. Consanguineous marriages were more frequent in this material than in the general population. Other congenital malformations were not observed more often in relatives of index patients than in the general population. Cleft lip with or without cleft palate meets the rules of polygenic inheritance. The incidence of posterior cleft palate was 2.48% in sibs, 0.41% in uncles/aunts and 0.48% in first cousins of index patients with posterior cleft palate. The aetiology of this malformation is presently not clear.