A posterior cerebral artery aneurysm presented as a seizure disorder in a 7-week-old infant. A small hemorrhage in the posterior thalamus was seen on CT scan. However, magnetic resonance imaging (MRI study) demonstrated an aneurysm and appropriate therapy was carried out. The incidence, natural history, etiology, and surgical treatment of these lesions are reviewed, and the usefulness of MRI in their diagnosis is emphasized.