Objectives: The purpose of this study was to investigate the advantages of endoscopic lateral osteotomy, to evaluate the effects of the periosteal elevation on ecchymosis and to search the reasons for the ecchymosis and the intranasal mucosal tears. Methods: On a randomly chosen side a subperiosteal tunnel was elevated, on the other side a subperiosteal tunnel was not elevated. Bilateral lateral osteotomies were performed. Internal nasal packing was not used. The post-operative care was similar for all patients. The patients were evaluated on the third post-operative day. The intranasal mucosal tears, the lateral osteotomy cut on the periosteal elevation side were examined using endoscopes. The degree of ecchymosis was determined by 2 other surgeons, who were unaware of the elevated side, using the grading system adapted from Hoffman et al. Results: The intranasal mucosal injury was seen in 16 of non-elevated side whereas it was 14 on the periosteal elevated side. Endoscopy showed the perforating arteries and elevation of the periosteum caused rupture and oozing from these arteries which might be a cause for ecchymosis. Ecchymosis was more severe on the side with subperiosteal tunnel in 6 patients; whereas only in 2 patients it was more severe on the side with no subperiosteal tunnel elevation while in 12 patients the degree of ecchymosis was the same on both sides. Conclusion: Endoscopy gives a great opportunity to visualize the lateral osteotomy site. Besides the lateral osteotomy cut, the arteries and the intranasal mucosal injury can be visualized. Periost elevation increases the severity of ecchymosis due to the rupture of the perforating arteries, not due to the trauma of the angular artery. Intramucosal tears do not increase the severity of the ecchymosis.