The purpose of this study was to estimate the genetic variance for alveolar bone height by means of the classic twin method and the study of monozygous twins reared apart. Panoramic radiographs were obtained from 120 pairs of adult twins (mean age = 40.4 years, S.D. = 10.4 years), for comparison of 62 pairs of monozygous twins reared together (MZT), 25 pairs of like-sexed dizygous twins reared together (DZT), and 33 pairs of monozygous twins reared apart (MZA). Mesial and distal bone heights were determined as a proportion of tooth length. A full-mouth bone score was computed for each twin by averaging these proportions from all measurable teeth. Between-pair (B) and within-pair (W) variances were computed for each twin group. The population variances (B + W) of the MZT and DZT twin groups were similar, which validated a basic assumption of the twin model. Intraclass correlations and heritability estimates were also computed for the reared-together and reared-apart twin groups. Boot-strap sampling was used to provide estimates and confidence limits for these values. The intraclass correlations for the twin groups were: MZT = 0.70, DZT = 0.52, and MZA = 0.55. The results of this study suggest that there is significant genetic variance in the population for proportional alveolar bone height.