We report a 57-year-old man with intraparenchymal pneumocephalus caused by ethmoid sinus osteoma. He had a history of severe allergic rhinitis, which caused him to frequently blow his nose, and he was referred to our hospital with headache and mild left hemiparesis. CT scans revealed a large volume of intraparenchymal air entrapped in the right frontal lobe related to an osteoma in the ethmoid sinus. The osteoma eroded the upper wall of the sinus and extended into the anterior cranial fossa. At operation, we observed that the osteoma had protruded intracranially through the skull base, disrupted the dura and extended into the frontal lobe. To our knowledge, this is the first report of a patient with intraparenchymal pneumocephalus caused by an ethmoid sinus osteoma.