We reported a case of posterior cerebral artery giant aneurysm with xanthochromic content. A 28-year-old male patient was admitted to our department suffering from lapses into unconsciousness once a month for the previous two years and a half. CT scan demonstrated a mass 2 cm in diameter in the right ambient cistern. The density of the inside of the mass was low, and that of outer border was high. The mass was not enhanced. MRI demonstrated that the intensity of the inside of the mass was low, and the outer part was iso-low in the T1 weighted image, while the inside was high and the outer part low in the T2 weighted image. Cerebralangiography showed an aneurysm of 5 mm in diameter at the bifurcation of the posterior cerebral artery (PCA) and the posterior inferior temporal artery. There was occlusion of the PCA. Accordingly, we diagnosed it as a calcified giant aneurysm with fluid-like content, and carried out surgery using the extended subtemporal approach. Inside the very stiff yellow-white wall of the aneurysm, there was only xanthochromic fluid without thrombus, and the structure of the aneurysm was cystic. The aneurysm was resected following neck clipping. After the operation, the patient's lapses into unconsciousness were able to be well controlled with anticonvulsants, and he left our department without any neurological deficit about a month afterwards. The histological findings of the specimen show the inner wall of the artery. Cases of giant aneurysms with xanthochromic content have rarely been reported. Only two are recorded, and this is the first case diagnosed using MRI.(ABSTRACT TRUNCATED AT 250 WORDS)