An extremely rare case of chondromyxoid fibroma of the skull base extending from middle fossa to posterior fossa is reported. The patient is a forty-seven year old woman who became paranoiac four months before admission and her gait was unsteady for one month. On April 27th, 1985, she fainted and was admitted to Kanto Rosai Hospital on April 30th. She was drowsy and disoriented. Neurologic examination revealed left pyramidal tract signs and impaired functions of right Vth (first and second branches), VIth and VIIIth cranial nerves. She was anemic and bleeding tendency due to thrombocytopenia was also recognized. Hematologist's diagnoses were myelodysplastic syndrome plus iron deficiency anemia. Neuroradiologic studies including CT scan, cerebral angiographies and MRI revealed a huge extra-axial skull base tumor extending from middle fossa to posterior fossa completely destroying the skull base. Stenosis of cavernous portion of internal carotid artery due to tumoral involvement was also noted. Surgical excision of the tumor was performed three weeks after the admission with the use of platelet transfusion. Transpetrosal transtentorial approach was performed removing the intracranial part of the tumor which was a semi-translucent jelly-like soft mass. The patient had a good post-operative course and improvement in the neurologic condition was recognized. Microscopically, the tumor showed marked lobular formation with small vessels and fibroblasts in the periphery of the lobule. The fibroblasts took stellate figures in the myxoid matrix at the center of the lobule. Chondroid pattern was also observed. Mitosis and nuclear atypism were not observed. Histological diagnosis was chondromyxoid fibroma.(ABSTRACT TRUNCATED AT 250 WORDS)