Computer-assisted medical imaging--transaxial computed tomography (CT) scans and three-dimensional surface reconstructions--was used to study the muscles of mastication and their osseous origins and insertions in 24 patients with untreated unilateral hemifacial microsomia (HFM). The relationship between the volume of a muscle of mastication and the shape and size of its origin and insertion in such patients varies widely. Comparison of mean volumes of specific muscles documents a statistically significant decrease among patients who have moderate to marked mandibular dysmorphology as compared with those with minimally dysmorphic mandibles. This study supports the hypothesis that the shape and size of the mandible are related to the muscles that originate and insert upon it. However, the variation among individual patients means that assumptions regarding muscle mass and, in turn, function cannot be made regarding an individual patient on the basis of osseous dysmorphology that has been demonstrated on skull radiographs alone.