The aim of this study was to evaluate if ipsilateral motor evoked potential (MIEP) elicited by transcranial magnetic stimulation (TMS) could provide neurosurgeons preoperatively with useful information regarding surgical procedure for patients with severe cerebral hemiatrophy or unilateral malformation. Thirteen epilepsy patients with severe cerebral hemiatrophy or unilateral malformation were studied before operation using MEPs recorded on bilateral abductor pollicis brevis (APBs) muscles, elicited by transcranial magnetic stimulation of the motor cortex. Ten subjects served as controls. Results: (11) no ipsilateral MEP responses were recorded in all the 10 healthy subjects; (2) in the 13 patients, the results of MEPs could be divided into four types. Type A: in 3 patients bilateral MEN were recorded when unaffected hemisphere was stimulated, while no responses were elicited when the affected hemisphere was stimulated. Type B: in another 3 patients, the MEPs were elicited from bilateral APB muscles when the unaffected hemisphere was stimulated, and the contralateral MEP was also elicited when the affected hemisphere was stimulated. Type C: in two patients contralateral MEP was elicited when the unaffected hemisphere was stimulated, while no MEP was induced in APB muscles of either side following the affected hemisphere stimulation. Type D: in the remaining 5 patients, contralateral magnetic MEN were elicited either when the affected or the unaffected hemisphere was stimulated. Patients of type A, B and C received hemispherectomy showed no significant permanent motor functional deficit. Among the total 8 patients, 7 patients got seizure free after the operation. Patients of type D showed minor muscle strength decrease after localized cortical resection. Three out of 5 patients of type D got seizure free after the operation. Ipsilateral MEP response might be useful for neurosurgeons to plan appropriate surgical procedure which helps avoid post-operative motor deficits. (c) 2009 Elsevier B.V. All rights reserved.