The patient is a 17-year-old female. She was suffering from dwarfism, irregular menstruation and obesity. Out of the dwarfism, there were no other neurological abnormalities. The serious clinical examinations were performed, and the cerebral angiography and the CT scan demonstrated the findings of the obstructive hydrocephalus. Namely, enlarged IIIrd ventricle and small IVth ventricle were observed with routine CT scan. Because these findings on CT scan, we thought the hydrocephalus was based on the aqueductal stenosis. The vertebral angiography showed stretched posterior medial choroidal arteries and compressed the basilar tip posteriorly and inferiorly. Moreover, the carotid-angiography demonstrated the following findings, unrolling of the anterior cerebral arteries laterally stretched bilateral lenticulostriate arteries laterally shifted sylvian groups of the middle cerebral arteries, and stretched bilateral internal cerebral veins. Judging from that angiographic findings and CT scan, we thought there would be cystic tumor in the IIIrd ventricle. However, we suspected the IIIrd ventricular tumor, we were not able to differentiated it from the enlarged IIIrd ventricle itself exactly. Because, it's density in CT scan was equaled with that the cerebrospinal fluid. According to above facts, we performed "Amipaque CT ventriculography through the ventricular catheter after V-P shunt. By "the Ampiqque CT ventriculography" we found exactly there was large cystic tumor in the IIIrd ventricle. Their manifestation and some differential diagnosis were discused.