Two cases of a disproportionately large communicating fourth ventricle (DLCFV) which developed after suboccipital craniectomy are reported. The first case was that of a 43-year-old female who sustained subarachnoid hemorrhage, and the second case was that of a 12-year-old male with medulloblastoma. Patency of the aqeductal canal and occlusion of the fourth ventricle outlets were demonstrated in both cases by metrizamide ventriculography, with intraventricular injection of contrast material. Clinical history was analyzed with repeated CT scan, and pathogenesis of DLCFV was discussed with review of the literature.