We examined the frequency and site of major complications after intranasal micro-endoscopic ethmoidectomy. Injury of the orbital periostium was significantly more frequent on the right than on the left side, but there was no side predominance for damage to the dura mater. Surgery on the lamina papyracea and the lateral wall of the sphenoid sinus is more dangerous on the side on which the surgeon stands during the operation. The use of the microscope, the endoscope and preoperative CT scan of the paranasal sinuses helps to avoid major complications.