Motor cortex activity was localized with positron emission tomography (PET) in 4 patients with large arteriovenous malformations adjacent to or undercutting the left primary motor cortex. Relative cerebral blood flow responses were measured during execution of a visually guided motor tracking task performed with the right index finger, hand, great toe, tongue, or eyes alone (control) and mapped onto each patient's corresponding magnetic resonance imaging (MRI) scan. The relative cerebral blood flow responses in the contralateral precentral gyrus, adjacent to each arteriovenous malformation, demonstrated a normal somatotopic distribution, similar to that in a control population. In the 3 patients with preserved motor function, responses were also present in the ipsilateral primary motor cortex, bilateral supplementary motor area, and ipsilateral anterior cerebellum, similar in location to those of a control population. In the fourth patient with a hemiparesis, responses were attenuated in the primary motor cortex, increased in the supplementary motor area, and absent in the cerebellum. The results demonstrate that PET cerebral blood flow mapping can localize motor cortex despite the presence of significant blood flow abnormalities in adjacent arteriovenous malformations. The method, particularly when combined with MRI, may be used in the planning of surgical, radiation, or embolization therapy.