WE REPORT A case in which three-dimensional (3-D) magnetic source imaging, using the combined techniques of magnetoencephalography and 3-D magnetic resonance imaging, was employed to localize precisely a structural lesion in the precentral gyrus. The lesion was primarily a hematoma located in the right frontal lobe and was associated with a cryptic arteriovenous malformation that had produced repeated seizures with progressive left hemiparesis. 3-D magnetic source imaging mapped the entire somatosensory homunculus and localized the hematoma in the precentral gyrus. Both this relationship and the deduced localization were confirmed by a cortical recording of somatosensory evoked potentials at the time of surgery. 3-D magnetic source imaging proved to be useful in localizing cortical lesions precisely and in determining preoperatively the best form of treatment for the peri-Rolandic lesions. (Neurosurgery 34:930-934, 1994)