The aim of this study was to evaluate quantitatively sagittal, skeletal and dental changes occurring during and after Herbst treatment and relate these changes to alteration in the occlusion. Lateral roentgenograms in centric occlusion were analysed. The sample consisted of 40 Class II, division 1 cases treated with the Herbst appliance for an average period of 7 months. The subjects were re-examined 6 and 12 months post treatment. The results revealed the following: during the treatment period all subjects were treated to Class I or overcorrected Class I or Class III dental arch relationships. Overjet and sagittal molar relationship improved by an average of 6.9 mm and 6.3 mm respectively. This was a result of a 2.2 mm greater mandibular than maxillary growth, a 2.3 mm lingual movement of the maxillary incisors, a 2.4 mm labial movement of the mandibular incisors, a 2.0 mm distal movement of the maxillary molars and a 2.1 mm mesial movement of the mandibular molars. During the post treatment period of 12 months the occlusion settled into Class I in all subjects. Overjet and sagittal molar relationship relapsed by an average of 2.2 mm and 1.7 mm respectively. About 90 per cent of the occlusal relapse occurred during the first 6 months post treatment. In 58 per cent of the subjects the occlusal relapse was exclusively a result of tooth movements while unfavourable maxillary and mandibular jaw growth contributed to the relapse in 42 per cent of the subjects. As Herbst treatment is performed during a relatively short period, the dentition will be in a state of instability after the appliance is removed. Post treatment retention as well as interocclusal adjustments with an activator are therefore recommended.