Background Obstructive sleep apnea syndrome (OSAS) is a sleep-related breathing disorder characterized by intense snoring and repetitive complete or partial obstructions of the upper airway during sleep together with daytime sleepiness. Several non-invasive therapeutical options exist, however, they do not offer a permanent improvement. Maxillomandibular advancement (MMA) surgery is a procedure which changes the upper airway in a permanent way. Objectives The investigators aim to develop a prospective database registering 3D volumetric changes of the upper airway and its anatomic subregions, the apnea-hypopnea index (AHI) and quality of life (QOL) of all consecutive patients eligible for MMA, performed by Dr. Neyt. Patient demographics, detailed virtual cone-beam computed tomography (CBCT) planning parameters, orthognathic surgery, polysomnographic and QOL data are being collected during consecutive visits within the framework of routine practice. Design A prospective, observational cohort study Study center General Hospital (AZ) Sint-Jan Brugge-Oostende Population The investigators would like to include all OSAS patients (AHI ≥ 5) requiring a MMA by Dr. Neyt starting from January 2015. Endpoints The investigators aim to collect data that could provide information about the advantages and disadvantages of the routinely performed 3D CBCT preoperative MMA surgery planning for OSAS patients, regarding 3D volumetric changes of the upper airway and its anatomic subregions and quality of life (QOL) in a subjective manner with the Epworth Sleepiness Scale (ESS) and the OSAS questionnaire and in an objective manner with evaluation of the apnea-hypopnea index (AHI). Duration In light of the continuous improvement of patient care, a database will be maintained from January 2015 onwards to enable registration of large-scale OSAS patient data. Conclusions Development of a database registering 3D CBCT planning, polysomnographic data and quality of life (QOL) of all consecutive patients eligible for MMA, will provide more information about potential patient, virtual planning and surgical factors influencing accuracy of MMA, and the associated biological benefits of this procedure on the upper airway volume, the AHI and general QOL. Moreover, registration of those results could function as a measurement of quality of care, or could be used for sample size calculation for future large multicenter prospective trials.