Idiopathic hypertrophic pachymeningitis is a rare disorder of unknown origin. It is a fibrosing inflammatory process that involves the dura mater. Herein are described 14 patients with idiopathic hypertrophic pachymeningitis; their clinical, laboratory and radiological findings, as well as their treatment, are analyzed. Neuropathological findings of six cases including two autopsied cases are also presented. The main clinical features were headache and cranial nerve palsies. Many patients had mild to moderate elevation of C-reactive protein, and three patients had perinuclear antineutrophil cytoplasmic antibody. The CSF in most cases showed inflammatory changes. Neuroimaging studies revealed diffuse or localized thickening of the dura, and MRI findings were key to diagnosis of this disorder. The clinical course was chronic. All patients were treated with corticosteroid and improved intially, but half of them experienced relapses. Two patients received surgical intervention. Pathological examination in two autopsied cases revealed diffuse thickening of the dura, especially in the posterior part of the falx cerebri and the tentorium cerebelli. Microscopic examination of the dura showed dense fibrosis with inflammatory cell infiltration composed mainly of lymphocytes. The cell infiltration was marked at the surface of the dura mater. One case had necrotizing vasculitis of the small arteries located in the dura and the cerebral surface. There were no giant cells, caseation necrosis, or epitheloid granuloma. Four patients underwent biopsy of the dura, and the pathological study showed non-specific inflammatory changes. The relationship of idiopathic hypertrophic pachymeningitis with connective tissue disease or vasculitis syndrome is discussed.