This 39-year-old man presented with a 6-month history of occipital headaches. Magnetic resonance imaging revealed an irregularly shaped fourth ventricle mass. One month after his initial presentation, he was admitted to the hospital with significant tumor expansion and clinical deterioration. A posterior fossa craniectomy was performed and the mass was resected. Histopathological analysis of this tumor showed central necrosis with associated edema in an otherwise typical and benign-appearing subependymoma. To the authors' knowledge, this is the first reported case of rapid, nonhemorrhagic expansion associated with necrosis in a previously asymptomatic subependymoma.