The heritability of dental characteristics has been systematically studied in animals, human populations, families, and twins, but not in twins reared apart. Under the assumption that environmental factors are no different for monozygotic twins reared apart than for dizygotic twins reared apart, the present study measured the genetic variance of several dental characteristics in twins reared apart. Ninety-seven subjects (44 twin pairs, three triplet sets) of mean age 40.6 years (S.D. 11.7) were examined over a six-year period by means of clinical and radiographic examinations, study models, and dental history questionnaires. Characteristics assessed retrospectively were: dentate status, treatment status, treatment/caries status, tooth size, malalignment, occlusion, and morphology. Data were analyzed by two-way ANOVA, intraclass correlations, heritability estimates, and concordance. There was statistically significant resemblance within monozygotic but not dizygotic pairs in the number of teeth present (p less than 0.001), percentage of teeth and surfaces restored (p less than 0.001), percentage of teeth and surfaces restored or carious (p less than 0.001), tooth size (p less than 0.001), and malalignment (p less than 0.009). Intercanine and intermolar arch width showed significant resemblance within both monozygotic (p less than 0.001) and dizygotic (p less than 0.01, p less than 0.05) pairs, whereas overjet and overbite showed no significant resemblance within pairs. Morphological features (Carabelli's trait and mandibular first premolar groove configuration) were more highly concordant in monozygous than in dizygous twins. This study provides new evidence for a marked genetic component to dentate status and dental caries experience and confirms previous reports of acknowledged inherited contributions to tooth size, malalignment, occlusion, and morphology.