To assess changes in bimaxillary morphology in patients with Class II division I malocclusion treated using Twin Block appliances (TBA), pre- and posttreatment lateral cephalographs of 100 patients seven to 16 years of age were compared. The patients were divided into four groups: (1) prepubertal boys (mean treatment time; 13.6+/-4.0 months), (2) adolescent boys (20.1+/-7.7 months), (3) prepubertal girls (12.3+/-1.4 months), and (4) adolescent girls (22.3+/-6.7 months). Five landmarks were digitized on pre- and posttreatment cephalographs for all groups, and the resulting configurations were analyzed using linear analysis, Euclidean Distance Matrix Analysis (WinEDMA), and Thin Plate Spline (TPS) analysis. Results of linear analysis indicated anteroposterior midfacial distances generally increased in all groups. In contrast, WinEDMA indicated that midfacial distances proportionately decreased in all groups treated with TBA. Graphical analysis with TPS showed anteroposterior compression of the configurations, most marked in the region of Prosthion. The affine components of the configurations following the TBA treatment indicated little deformation, but the nonaffine components demonstrated posterior displacement of Prosthion. Thus, improvements in facial balance following TBA treatment are associated with a relative restriction of anterior displacement within the midfacial complex as well as maxillary dento-alveolar effects. Orthodontic treatment methods exerting growth-inhibitory effects within the maxilla may be useful in the correction of Class II division I malocclusions.