A 76-year-old female presented with a large upper basilar artery (BA) aneurysm causing obstructive hydrocephalus after coil embolization manifesting as diplopia. Magnetic resonance (MR) imaging and MR angiography showed a large BA top aneurysm. Coil embolization was performed. More than 6 months after the first coil embolization, the aneurysm had re-grown and we performed a second coil embolization. Soon after that, obstructive hydrocephalus at the aqueduct of the midbrain occurred. MR imaging was performed to evaluate whether there was enough space at the prepontine cistern for a third ventriculostomy and also to verify the posterior direction of the aneurysm growth because of the risk of rupturing the aneurysm during the operative procedure. Then, we performed an endoscopic third ventriculostomy (ETV) via a left-sided approach. We could easily identify the infundibular recess, mamillary bodies, and tuber cinereum in the third ventricular floor. We detected a pulsating upper BA aneurysm that appeared to have caused a reddish color change in the right mammillary body and the right side of the thalamus and midbrain. If there is sufficient space in the prepontine cistern for the surgical procedure, ETV is a good choice for the treatment of obstructive hydrocephalus associated with cerebral aneurysms.