Objective: To examine the surgical correction methods for treating cases of severe mandibular hypoplasia accompanying obstructive sleep apena syndrome (OSAS). Methods: Sixteen cases of severe mandibular hypoplasia were studied in which OSAS was documented by polysomnography (PSG) and cephalometric study. The obstructive site was at the base of the tongue. Surgical procedures such as temporomandibular joint (TMJ) reconstruction and bimaxillary, chin, and hyoid bone advancement were performed to improve each patient's profile, function, and occlusion, and to treat the OSAS. Results: There were great improvements in patient's sleep and daytime quality of life. The pre- and postoperative changes of most PSG values and some cephalometric values (SNB, PAS) were statistically significant. Conclusions: Severe mandibular hypoplasia can cause not only abnormalities in profile and occlusion but also OSAS. The evaluation of OSAS and its treatment effects depend on PSG. It is also very important to confirm the obstructive site in the upper airway by cephalometric study and fiberoptic endoscopy. Orthognathic surgery procedures can advance the maxillary, chin, and hyoid bone, and expand the upper airway simultaneously. These procedures can treat OSAS. Cases of TMJ ankylosis with OSAS should be treated step by step. (CJDR 1999;2(3):59-64