A 52-year-old woman was admitted to our hospital because of nausea, headache, ataxic gait, and memory disturbance. CT scans and MRI showed a markedly enhanced lesion in the dura mater of the posterior fossa, edema in the right cerebellar hemisphere, occlusion of the transverse sinus, and hydrocephalus, suggesting hypertrophic pachymeningitis. Chronic otitis media and mastoiditis were also noted. Subjective complaints diminished after steroid administration. The patient underwent right suboccipital craniectomy and biopsy. The dura mater was markedly thickened, and the cerebellar surface was edematous. Histological examination revealed the thickened dura mater to be infiltrated by chronic inflammatory cells of forming lymphatic follicles and accompanied by Langhans giant cells. Inflammatory cells were not found in the cerebellar parenchyma, but were present in the subarachnoid space. Acid-fast stain, PAS and Grocott stain were negative. The cause of the hypertrophic pachymeningitis in this patient, may have been related to the chronic otitis media, but the patient's clinical course suggested that it might represent so-called "idiopathic hypertrophic pachymeningitis".