The authors present a case of a man, 38 years old, who presented since a month a right hemiparesia, without aphasia. The paresia in the right side began with a violent headache, which would not give way to the analgesics, but little by little, disappeared completely. A careful general examination did not show any cause capable of explaining the hemiparesia. Ophthalmologic and spinal fluid examinations were absolutely normal. The plain radiograms shows a opacity of compact consistency in the frontal sinus and a voluminous image occupying the largest part of the left side of the skull. The sagital tomographic sections revealed erosion of the posterior wall and floor of the frontal sinus in continuity with the air collection. Once the patient was operated on, the integrity of the dura mater was evident. A few days after the intervention, the voluntary movements of right side of the body had already improved considerably, and a month later the neurologic examination was entirely negative. The removed tumor was an osteoma eburneo. After studying the otorhinolaryngologic aspects of the osteoma of the frontal sinus the authors consider its eventual complications. If the tumor grows lateral wards, it may invade the orbit, causing exophtalm, proptosis and diplopia; expanding backwards, it may erode the posterior wall of the frontal sinus causing an extradural pneumatocele, as occurred in the case registered in the present paper; it may also successively perforate the dura mater (sub-dural pneumatocele), invade the brain tissue (intra-cerebral pneumatocele), and even communicate with the lateral ventricle (ventricular pneumatocele). These intracranial collections (pneumocephalus) are exceedingly rare eventualities in a rare illness. As a matter of fact, until 1941, 321 cases of "osteomae" had been registered, of which only 8 had complications of pneumocephalus (2 extra-dural, 4 intra-cerebral and 2 ventricular). Besides, these neurologic complications of "osteomae" may get worse with inflammation brought from the infected paranasal sinus. Comparing the cases of traumatic sub-dural pneumatocele with those determined by sinusal "osteomae" the authors emphasize the inconstancy and transitoriness with which, in the last ones, the rhinorrhea is present. The naso-frontal duct must be permeable to allow the formation of intracranial aerial collection; evidently if this one has been previous obliterated by the expansion of the tumor, the pneumatocele cannot be produced. The authors explain the hemiparesia presented by the patient, as the result of a compression by the air collection, through the dura mater and the subjacent plans, on the left cerebral hemisphere.