A cephalometric evaluation of the changes in the horizontal and vertical placement of the hyoid bone and of those in the position of the head over the cervical spine after surgical mandibular advancement was undertaken. Seven linear and one angular measurement were investigated in 60 patients, 17 males and 43 females, before and one year after surgical mandibular advancement. The hyoid bone moved forwards horizontally in 78 per cent of the subjects, backwards in 17 per cent, and in 5 per cent of patients it retained its pre-operative position. It moved vertically downwards in 32 per cent of subjects, and in 63 per cent it moved upwards and closer to the body of mandible. The amount of horizontal and vertical change of the hyoid bone was associated with the corresponding change of the mandible after surgery. The vertical change was more distinct in females compared with males. There was variation in the position of the head over the cervical spine; it showed extension in 26.7 per cent of the sample, flexion in 71.7 per cent, and remained the same in only 1.6 per cent after surgery. The results show that with surgical mandibular advancement the hyoid bone follows mainly the advancement of the mandible and moves closer to the body of the mandible. However, there are variations in the changes of hyoid bone and head position that are difficult to predict.