This retrospective clinical audit identified the incidence of inferior alveolar nerve dysfunction following mandibular sagittal split osteotomies for a period of 1 year at the Derby Royal Infirmary. The clinical notes of 50 consecutive patients treated with a bilateral sagittal split osteotomy over the last 5 years were used to collect information on the following: age, sex, type of mandibular movement (advancement or setback), recorded nerve damage during surgery, good or ad split, and the presence of nerve dysfunction at representative postoperative intervals, up to 1 year.One hundred procedures were essentially audited, as the surgery was carried out on both sides. The mean age of patients undergoing this procedure was 23 years. Thirty-four patients were female and 16 were male. Thirty-three patients underwent advancements and 17 underwent setbacks. The only nerve damage recorded was stretching; this occurred in 11 procedures. The incidence of nerve dysfunction recorded postoperatively was: 22% to 78% immediately, 29% to 61% after 1 week, 23% to 45% after 1 month, 8% to 34% after 3 months, 5% to 26% after 6 months, and 22% to 78% after 1 year. The findings from this clinical audit show that the incidence of inferior alveolar nerve dysfunction at the Derby Royal Infirmary is comparable to, if not less than, what was reported in previous studies. Hence, there is evidence of a reasonably high standard of care taken to prevent this damage, and no changes in the way the surgical procedure is carried out is indicated at present. However, this clinical audit did highlight the lack of documentation and standardization of postoperative assessment for nerve dysfunction. There may be a need for the introduction of a pro forma assessment system to the department. (Int J Adult Orthod Orthognath Surg 2001;16:266–271)