OBJECTIVE: To study the effects of jaw advancement in treating micromandibular deformity associated with obstructive sleep apnea syndrome (OSAS) by ramus osteotomy and genioplasty. METHODS: From April 1998 to February 2002, 12 patients with micromandibular deformity associated with OSAS (aged 14-36 years, 7 females and 5 males) were treated. Invert "L" shape ramus osteotomy and inverted replantation of posterior segment of ramus were performed to reconstruct the TMJ with the jaw advancement and genioplasty at the same time in 7 cases; mandibular angle osteotomy, bone grafts and genioplasty in 3 cases; and the jaw advancement by ramus sagittal osteotomy and genioplasty in 2 cases of the first branchial arch syndrome. RESULTS: The follow-up period was 6 months to 4 years. All the patients gained good appearance and had the distance of opening movement over 3.0 cm. Micromandible and facial asymmetries were corrected satisfactorily. The ratio of SaO2 was ascended from 82%-92% (preoperation) to 97%-99% (postoperation). OSAS was relieved. CONCLUSION: The jaw advancement by ramus osteotomy and genioplasty for treating micromandibular deformity associated with OSAS can correct the maxillofacial deformities and enlarge the upper airway space to relieve OSAS. This method has achieved satisfactory result.