A multifactorial model based on an underlying continuous distribution of tooth size, with thresholds determining hypodontia and supernumeraries, has been proposed [1]. Our aim is to investigate this model by comparing tooth morphology of affected patients and their first degree relatives with normal controls. An image analysis system [2] was used to measure teeth on study models of controls, patients with hypodontia of varying degrees and location, first-degree relatives of hypodontia index cases, and patients with supernumerary teeth in the maxillary incisor region. Mesio-distal crown size in hypodontia patients was smaller than controls, and this difference was significant for all tooth types. There was a general pattern: the more severe the hypodontia, the smaller the size of the tooth formed. Patients with supernumerary teeth had permanent maxillary central and lateral incisors and canines that were significantly larger in mesio-distal width than controls. The maxillary central incisors also differed in taper in supernumerary patients compared to controls. Hypodontia prevalence was higher in first-degree relatives (22%) of hypodontia index cases than in the general population (4.4%), and unaffected relatives had smaller teeth than controls. Thus, there were generalized and localized effects within the dentition, and these findings are compatible with the statistical expectations of the proposed multifactorial model.