Isolated trigeminal sensory loss secondary to a distal anterior inferior cerebellar artery aneurysm: case report. A previously healthy 25-year-old woman suddenly developed right-sided facial numbness and a headache. The neurological examination was within normal limits with the exception of meningismus and right-sided facial sensory loss. A computed tomographic scan and a magnetic resonance imaging study demonstrated an acute hematoma in the right cerebellopontine angle. A 4-vessel cerebral angiogram revealed no abnormalities. Posterior fossa exploration disclosed a large, partially thrombosed, fusiform anterior inferior cerebellar artery aneurysm, which indented the pons at the trigeminal root entry zone. The aneurysm was excised, and the patient made an excellent recovery. She was left with a persistent trigeminal sensory deficit. Anterior inferior cerebellar artery aneurysms are rare lesions that generally present with a cerebellopontine angle syndrome; occasionally, facial sensory loss is also a feature. Isolated trigeminal sensory findings, as illustrated in this case, are extremely unusual in posterior fossa vascular lesions. 