Ethmoid operation may cause rather serious complications than any other sinus operations due to its anatomical complexity. Especially, orbital and intracranial complications such as orbital cellulitis, ocular palsy, optic nerve damage, CSF rhinorrhea, meningitis, brain abscess and cavernous sinus thrombophlebitis may result from damage on the orbit and anterior skull base. So it has been stressed that the importance of its thorough anatomical and surgical experiments prior to ethmoid operation. Authors experienced 5 cases of complications following transantral ethmoidectomy from 1967 to 1979, they were 2 cases of meningitis and each case of ocular palsy, optic nerve damage and CSF rhinorrhea. We present them briefly, analize their causes and discuss the surgical anatomy of ethmoid and high points of ethmoid operation with literature review.