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The aim of this study was to compare the effectiveness of 2-phase (a first phase with Twin Block therapy immediately followed by a second phase with fixed appliance therapy with nonextraction) and 1-phase (fixed appliance therapy with extraction of 4 first premolars) treatment of class II division 1 malocclusion.Two groups of successfully treated subjects, aged 10 to 14 years with class II division 1 malocclusion, were evaluated: 2-phase group (n = 70) and 1-phase group (n = 76). Pretreatment and posttreatment lateral cephalograms were traced manually and subjected to Student t test.In the 2-phase group, the forward positioning of the mandible (Pg-Olp) was less than that of the extraction group. The sagittal mandibular forward growth (Pg-Olp) was, on average, 2.39 ± 0.4 mm in the 2-phase group and 4.56 ± 2.51 mm in the 1-phase group (P = 0.008, P < 0.05). In the vertical plane, the increases in lower facial height and mandibular plane angle were greater in the 2-phase treatment. In the 2-phase group, the lower anterior teeth proclined (Ii-MP) by 7.3 ± 2 degrees, and in the extraction treatment group, it changed −4.1 ± 3.6 degrees. The 1-phase group contributed more sagittal orthopedic effects than did the 2-phase group.The proclination of the mandibular incisors in the 2-phase group might restrain the mandibular bone from growing enough and cause the backward and downward rotation of the mandible.