There is increasing interest in staged corpus callosotomy for intractable generalized epilepsy. At the first procedure, a portion (usually the anterior two-thirds) of the corpus callosum is sectioned. If seizures persist, completion of callosotomy or alternative treatment approaches can be considered. It is obviously important to ascertain that the desired extent of callosotomy was in fact accomplished at the time of initial operation. Our experience and the published literature indicate that the surgeon's impression at operation can be erroneous. We describe a technique of determining extent of corpus callosotomy during the procedure. The magnetic resonance imaging (MRI) scan in the midsagittal plane is used to select the desired extent of callosotomy. That point on the corpus callosum is characterized using simple planar geometry in relation to three anatomic landmarks in that same plane: the glabella, the inion, and the bregma (midline intersection of the coronal suture). The same point along the corpus callosum can then be located on a lateral skull x-ray using these same three anatomic landmarks. At surgery, an intraoperative lateral skull x-ray is obtained with a marking clip, thereby verifying the actual extent of callosotomy. We have verified the reliability of this scheme in 5 callosotomy procedures and have used this technique for intraoperative localization of midline and parasagittal targets in another 7 cases (3 tumors, 2 aneurysms, and 2 placements of interhemispheric subdural grids). In addition, we reviewed corpus callosum topography on 25 randomly selected MRI scans.(ABSTRACT TRUNCATED AT 250 WORDS)