BACKGROUND: Unilateral craniofacial microsomia is characterized by soft-tissue and bony deficiencies. Mandibular distraction osteogenesis can be used to augment the hypoplastic skeleton and improve facial symmetry. The aim of this study was to determine how the vector of unilateral mandibular distraction affects treatment outcomes. METHODS: A retrospective chart and radiographic review was conducted of all patients treated with external mandibular distraction osteogenesis between October of 1990 and February of 2004 (n = 185). A subset of 42 patients underwent primary unilateral, uniplanar, external distraction, and 13 patients were found to have satisfied inclusion criteria and had adequate predistraction and postdistraction lateral and posteroanterior cephalograms. Cephalometric tracings were made and multiple points and planes were assessed before and after distraction. RESULTS: A strong correlation was noted between the vector of distraction and the movement of the mandible. A horizontal vector of distraction resulted in minimal increase in ramal length but a marked shift in the mandibular midline (r = 0.68, p < 0.05). In contrast, a vertical vector of distraction resulted in marked mandibular ramus lengthening but minimal mandibular midline shift (r = 0.73, p < 0.05). Mathematical formulas were derived to correlate the distraction vector and mandibular movements to improve preoperative planning. CONCLUSIONS: Successful distraction is dependent on accurate preoperative planning and prediction of outcomes. This study demonstrates a predictable relationship between the vector of unilateral distraction and the mandibular response.