BACKGROUND The main procedure specific complication of sagittal split osteotomy of the mandibular ramus (SSOMR) is inferior alveolar nerve (IAN) injury. This can be produced by poor intraoperative visibility of the IAN due to bone bleeding. In our centre, mandibular nerve blocks (MNBs) are usually performed for intra- and post-operative analgesia. We observed that MNB seems to decrease intraoperative bleeding and thus to improve IAN visibility. Our study was performed to evaluate the effect of MNB on intraoperative bleeding during SSOMR and, secondarily, on the duration of this procedure. METHODS Patients scheduled for bilateral SSOMR under general anaesthesia were prospectively randomized into two groups. The Block group received bilateral MNB (5 mL ropivacaine 0.5% for each block) and the Control group sham MNBs (bilateral cutaneous puncture, without block). The operations were carried out under a standardised general anaesthetic. Post-operatively, the surgeon, blinded to group allocation, assessed intraoperative bone bleeding using a Numerical Rating Scale (NRS) (0: no bleeding, perfect visibility to 100: major bleeding, no visibility). Osteotomy duration, intraoperative anaesthetic requirements, and pain score in the recovery room were also recorded. Results are expressed as median [25-75th percentiles]. RESULTS Nineteen patients were included in each group. Osteotomy under MNB had a decreased intraoperative bone bleeding score compared with controls (20 [0-40] versus 55 [20-80], p=0.0002). They had a dry surgical field more frequently (29% versus 5%, p=0.01), and a shorter mean time for the osteotomy (15 [12.25-17.75] versus 17.5 [15-21]min, p=0.009). Block patients had reduced intraoperative opioid consumption (770 [678-1430] versus 2310 [1908-3058]mcg of remifentanil, p=0.0001), and lower pain scores in the recovery room (0 [0-2] versus 3 [1-5], p=0.12). CONCLUSIONS MNB decreases intraoperative bone bleeding during SSOMR under general anaesthesia. Three hypotheses to explain this result are discussed.