A case of a giant aneurysm of the proximal segment of the posterior cerebral artery is reported. Complete neuroradiologic (computed tomography scan, angiography, magnetic resonance imaging) and pathological studies were performed. This type of aneurysm is extremely rare and may be difficult to differentiate from a cerebral tumor, both clinically and on computed tomography scan. Vertebral angiography is usually necessary to make, or confirm, the diagnosis.