The aim of this study was to examine the effects of mandibular setback surgery on pharyngeal airway space and respiratory function during sleep. The subjects were 22 patients in whom mandibular prognathism was corrected by bilateral sagittal split ramus osteotomy; either one jaw or two jaw surgery. Polysomnography was performed before surgery and 6 months after surgery, and the apnea hypopnea index (AHI) and arterial oxygen saturation during sleep were measured to assess respiratory function during sleep. Morphological changes were studied using cephalograms taken immediately before, a few days after and 6 months after surgery. As a control, 10 subjects without sleep-disordered breathing underwent the same examinations. AHI was not changed significantly after surgery, although two patients were diagnosed with mild obstructive sleep apnea (OSA) syndrome after surgery. They were not obese, but the amounts of mandibular setback at surgery were large. In conclusion, a large amount of mandibular setback might inhibit biological adaption and cause sleep-disordered breathing, and it might be better to consider maxillary advance or another technique that does not reduce the airway for patients with skeletal class III malocclusions who have large anteroposterior discrepancy and/or maxillary hypoplasia.