The authors present the case of a 22-year-old man with an unruptured arteriovenous malformation (AVM) in which an intranidal aneurysm had grown in the course of 3 months and was complicated by perifocal brain edema. A left parietal AVM was incidentally diagnosed on magnetic resonance (MR) imaging. No aneurysms were noted on cerebral angiograms obtained simultaneously. Three months later, T2-weighted MR imaging revealed perifocal brain edema (increased signal intensity in the brain parenchyma adjacent to the nidus). An aneurysm-like signal void was demonstrated in the center of the high-signal area, but no previous hemorrhages could be detected. Angiographic studies revealed an intranidal aneurysm 4 mm in diameter projecting anterolaterally from the nidus. Surgical removal was performed without incident, and no neurological deficits or postoperative complications were observed. An MR image obtained 2 weeks postsurgery revealed complete resolution of the perifocal brain edema. To the authors' knowledge, this is the first reported case of an unruptured AVM in which an intranidal aneurysm with perifocal brain edema developed rapidly (within a few months).