A 58-year-old man presented with a sudden onset gait disturbance and urinary incontinence. Magnetic resonance (MR) imaging and MR angiography showed a fusiform basilar artery aneurysm, 2 cm in diameter and 5 cm long. The aneurysm was thrombosed except the ectatic basilar artery flowing through it. Compression of the cerebrospinal fluid (CSF) pathways caused obstructive hydrocephalus with resultant transependymal CSF leakage. Conventional angiography missed the aneurysm. A one-sided ventriculoperitoneal shunt was placed to treat the patient's hydrocephalus. The patient's symptoms improved after surgery. MR imaging and MR angiography were useful noninvasive methods of detecting the aneurysm, defining its relationship with surrounding structures, and determining the cause of the hydrocephalus.