Various attempts to describe the skeletal characteristics of unilateral craniofacial microsomia have been made with the use of cephalometric and panoramic roentgenograms. Previous studies have been only descriptive in nature. To date, a detailed (quantitative) cephalometric analysis of the mandibular deformity has not been reported. The purpose of this study was to describe the skeletal jaw deformity by means of cephalometric landmarks in the lateral view. The patient population consisted of sixteen boys and eight girls who ranged in age from 6 to 16 years. They were compared to the University of Michigan normal control population for the following measures: gonial angle, mandibular plane angle, overall oblique length of the mandible, ramal height, and body length. The affected side showed a larger gonial angle and mandibular plane angle. The oblique length of the mandible (Cd-Gn) was decreased on both sides, as were ramal height and body length. Paradoxically, body length appeared shorter on the unaffected side than on the affected side. This paradoxical observation could be attributed to a shift of the mandible in relation to the midsagittal plane of the cranial base, the film cassette, and the path of the x-ray beam. Observation of the mandible in the basilar cephalogram explained the geometry of the projection error found in the lateral view. Similar projection errors exist for patients with other types of craniofacial asymmetry. It is suggested that two radiographic views, orthogonal to each other, should be used to define the x, y, and z planes for studies of craniofacial abnormality.