Plaster study models and intraoral and extraoral radiographs of 165 twin pairs (99 mono and 66 dizygotic) were used to investigate: 1. the frequency of hypodontia of permanent teeth in twins, 2. the concordance-discordance rate of hypodontia in monozygotic and dizygotic twins, 3. the empirical risk of hypodontia occurring in twins. Out of 165 sets of twins, 14 (9 mono and 5 dizygotic) or 8.5% (5.5% monozygotic and 3.0% dizygotic) had hypodontia. This was within the frequency range of the general population (from 2.3% to 9.6%). Second maxillary incisors were the teeth most frequently absent or hypoplastic (peg-shaped). This was found in 5 monozygotic and 3 dizygotic sets of twins. It was followed by agenesis of the lower second premolars (in 5 pairs). Hypodontia of the remaining teeth was a rare finding. One member of a monozygotic pair showed agenesis of the left maxillary canine and the other agenesis of the right (a mirror-image similarity). The simultaneous appearance of missing teeth (hypodontia) and supernumerary teeth was found in two pairs of twins (1 monozygotic and 1 dizygotic). Monozygotic twins showed a significantly higher concordance rate for hypodontia than did the dizygotic. Out of 9 monozygotic pairs, 8 (88.9%) were concordant and only one pair discordant for the trait. On the other hand, all 5 dizygotic pairs were discordant for hypodontia. This suggests a high genetic component in the trait. Calculating the risk of hypodontia appearing in twins on the basis of the 14 pairs (both monozygotic and dizygotic) considered here showed that if one member of a pair of twins is affected by hypodontia, the risk of the other twin having the same abnormality increases to 57%. However, if one of a pair of monozygotic twins has the trait, the risk that the other will also have it is increased to 89%. Should however the pair be dizygotic, the risk of the other being affected is reduced to zero.