Marked non-communicating hydrocephalus may rarely cause ventricular rupture producing either a dilated cystic cavity (ventricular diverticulum) or communication between ventricle system and subarachnoid space (spontaneous ventriculostomy). Ventricular diverticulum has been believed to be collection of cerebrospinal fluid which escaped beneath the pia mater after rupture of ependymal layers and cerebral parenchyma. We proposed herein to report a case of subtentorial ventricular diverticulum which accompanied with choroid plexus papilloma of the lateral ventricle. A nine year-old girl admitted to our hospital complaining of clumsiness of hands and walking, disability of reading, headache and vomiting. The neurological examination revealed alexia, papilledema, anisocoria, righ hemianopsia, weakness of right upper limb, and cerebellar ataxia. CT brain scan showed a large high density area at the trigone of the left lateral ventricle with non-communicating hydrocephalus and an extra-axial low density area in the posterior fossa. The medial space of the left trigone was especially ballooned and is just shifted above incisura tentoria by the tumor. The intraventricular tumor was totally removed by operation and proved to be benign choroid plexus papilloma microscopically. The subtentorial mass was confirmed to be a cyst contiguous to the medial trigone of the lateral ventricle, namely ventricular diverticulum. Three special features were recognized in this case. The first, this was the first example of ventricular rupture accompanied with tumor in the lateral ventricle reviewing all reports of both ventricular diverticulum and spontaneous ventriculostomy. The second, this ventricular diverticulum was not produced merely by hydrocephalus alone but mainly by direct effects of the tumor.(ABSTRACT TRUNCATED AT 250 WORDS)