The authors experienced a case of histiocytosis X with a large intracranial mass resulting in a convulsive seizure. The patient showed left exophthalmos and a skin rash one year and two months after birth. Histiocytosis X was diagnosed from a skin biopsy, and predonine, endoxan and vincristine were administered. The rash disappeared, but the exophthalmos remained. At the age of two years and nine months, punched-out lesions appeared in the skull and 4,000 rads of radiation was applied. Thereafter, the exopthalmos persisted but there was no particular problem in the course. However, a convulsive seizure with fever suddenly appeared at nine years and ten months of age and the patient was hospitalized. At the time of admission, the general condition was good and there were no abnormalities in neurological tests. In neuroradiological examinations, a calcified and poorly vascularized mass 8 cm in maximum diameter was found to occupy the left middle cranial fossa. Chondrosarcoma was strongly suspected from these findings, but there was also symmetrical thickening of bone cortex in the peripheries of the long bones of the extremeties which appeared to be the recovery process from bone destruction caused by histiocytosis X. Therefore, the formation of an intracranial mass by histiocytosis X was diagnosed and surgery was performed. When left osteoplastic fronto-temporal craniotomy was performed, the mass was found to be raising the temporal lobe and it could be easily separated from the surrounding tissue. However, these was firm adherence to dura mater of the middle cranial fossa (especially that of the superior orbital fissure). Histologically, there were many cells with small nuclei, no polymorphism, abundant and clear cytoplasm which were darkly stained and slightly atypic. These findings matched those for histiocytosis X. Cases of histiocytosis X rarely show symptoms of the central nervous system or infiltration of the central nervous system. Only 31 such cases were seen in the literature investigated by the authors. Neurological symptoms include pyramidal symptoms such as hemiparesis and impairment of the cranial nerves, particularly paresis of the optic, trigeminal, facial and acoustic nerves. Convulsive seizures were seen in only five cases including the one reported here. It is also rare for intracranial masses to be formed in cases of histiocytosis X and only six cases, including the authors', have been found with masses of a maximum diameter of more than 5 cm.