The purpose of this study was to make a quantitative evaluation on lateral cephalograms of skeletal and dental changes contributing to Class II correction in Herbst and Bass appliance therapy. From two male patient samples treated with either the Herbst appliance (n = 72) or the Bass appliance (n = 32) 18 pairs of subjects were selected. The subjects within each pair had to be in the same growth period at the time of therapy and to exhibit a similar pretreatment sagittal (ANB angle) and vertical (ML/NSL angle) jaw base relationship. Lateral cephalograms in centric occlusion taken before treatment and after 6 months were analysed according to the method of Pancherz (1982a). After six months of treatment the Bass appliance seemed to have a greater influence on mandibular jaw base position than the Herbst appliance, although no statistical significant differences existed between the treatment groups. The correction in overjet and sagittal molar relationships was more complete in the Herbst than in the Bass patients. This was due to greater dental changes (distal movement of the maxillary molars, proclination of the mandibular incisors) taking place in the Herbst subjects.