A 39-year-old male was admitted because of slowly progressive disturbance of consciousness, diplopia, and ataxia after laparotomy. Ventriculoperitoneal shunting and removal of an arteriovenous malformation had been performed previously. Neurological examination on admission revealed drowsiness, rotatory nystagmus, Parinaud's sign, and truncal ataxia. Computed tomography scan revealed extraordinary dilatation of the fourth ventricle compared with other dilated ventricles, and old low-density areas in the cerebellar hemispheres. After an external ventricular drainage (EVD) was inserted, all the ventricles decreased in size and the symptoms disappeared. The authors confirmed the patency of the aqueductal canal. One week later, the EVD was replaced by a ventriculoperitoneal shunt. A disproportionately large, communicating fourth ventricle (DLCFV) should be differentiated from an isolated fourth ventricle, which consists of marked enlargement of the fourth ventricle with obstruction of both the inlet and outlet of the fourth ventricle. The authors propose the importance of the fragility to pressure of the brain parenchyma and cerebellar hemispheres around the fourth ventricle as the mechanism of producing DLCFV.