The authors report a unique case involving a 2-year-old child with idiopathic moyamoya disease who presented with cerebral infarctions and seizures. On initial evaluation, computerized tomography (CT) showed a left parietal infarct and angiograms demonstrated early moyamoya disease with no evidence of arteriovenous malformation (AVM). Approximately 9 years later, angiography and magnetic resonance (MR) imaging revealed an AVM centered on the same region of the left parietal lobe. Angiographic, CT, and MR images are presented that demonstrate the progression of moyamoya disease and de novo development of the AVM in the infarct site. The possible role of angiogenesis in the etiology of acquired AVMs and moyamoya disease is discussed.