The position of the bolus in persons with mandibular prognathism was examined using a cineradiographic method before and after surgical correction of the prognathism. The pre-operative study included 30 persons, 18 women and 12 men. The test foods were bread and toffee. The results showed that the upper incisal region participated more and the premolar and molar regions less frequently in the chewing act than in other patient groups examined earlier with the same method. This was especially the case in persons with pronounced mandibular prognathism. It was concluded that these patients may have a poor masticatory ability.The post-operative registration was carried out on 23 of the patients. The results showed a more«normal»chewing pattern with a higher participation of the molar region. Due to this change the possibilities for a good masticatory function was considered increased after operation, but whether the chewing efficiency was improved could not be decided from the present study.