Objective To investigate the pathogenesis pathology,CT diagnosis and differential diagnosis of hydranencephaly.Methods CT manifestations and clinical data of 9 cases with hydranencephaly were analysed retrospectively.Results CT findins of 9 cases were almost defects of bilateral cerebral hemisphere and ventricle.Only a little remainder of occipital,frontal and/or temporal lobes were appearence.The structure of subtentorium was normal.Conclusion CT scan directly shows all appearances of hydranencephaly and is great valuable for diagnosing this disease.