PURPOSE: To report two infants with acquired, isolated third nerve palsies attributable to intracranial cerebrovascular malformations. DESIGN: Observational case report. METHODS: Two patients are described. Each was examined in a university-based pediatric ophthalmology and neuro-ophthalmology practice. RESULTS: An 8-month-old child presented with a pupil-involving partial left third nerve palsy because of a partially thrombosed fusiform aneurysm of the left internal carotid artery. A 3-month-old infant developed a right third nerve palsy from a giant arteriovenous fistula arising from an M2 branch of the right middle cerebral artery. CONCLUSION: Patients younger than 8 months and 3 months with acquired, isolated third nerve palsies resulting from intracranial cerebrovascular malformations could not be found in a MEDLINE search. In conclusion, at even this young age, acquired, isolated third nerve palsies may be the initial manifestation of an intracranial aneurysm or fistula. Magnetic resonance imaging (MRI) and MRI-angiography were adequate for detecting these processes.