Acute expansion of an intracranial aneurysm is an extremely rare event. In this report, we describe the unusual radiographic documentation of acute expansion of a posterior communicating artery (PCOM) aneurysm from 4 to 8 mm, resulting in complete oculomotor nerve palsy. A 40-year-old man was admitted to our institution with a 3-week history of persistent occipital headaches but was otherwise neurologically intact. CT angiography demonstrated a 4 mm left PCOM aneurysm. Digital subtraction angiography performed several hours following presentation demonstrated an expansion in aneurysm size to 8 mm with new-onset complete oculomotor nerve palsy. The aneurysm was successfully obliterated by clip ligation. Although growth in aneurysm size is often a progressive phenomenon, acute expansions of aneurysms can occur.