The frequency and expression of the Carabelli trait among 36 twin pairs discordant or concordant for cleft lip and/or palate were investigated. Four of 10 monozygotic (MZ) and 24 of 26 dizygotic (DZ) pairs of twins were discordant for clefts. The frequency of the trait on the first upper molars of the noncleft subjects was 79%. It varied from 54% to 61% in the subjects with clefts, and the frequency of the trait decreased with increasing extension of the cleft. The occurrence of the trait was bilateral in more than 90% of the MZ and the DZ subjects. MZ twins had a higher concordance rate for the corresponding molar comparisons and for the cross-twin comparisons than did the DZ twin pairs. The corresponding polychoric correlations for MZ twin pairs were estimated to be 1.0, and for the DZ twin pairs estimates ranged from 0.4 to 0.5. Thus, genetic factors appear to control the Carabelli trait, and the heritability seems to be high even in our cleft lip and/or palate twins.