We performed a prospective study using preoperative magnetic resonance imaging to identify hippocampal formation atrophy in 15 consecutive patients with intractable partial epilepsy who had undergone a stereotactic resection of an extrahippocampal temporal lobe foreign-tissue lesion. A stereotactic lesionectomy was performed in all patients, i.e., only the imaging-defined lesion itself was resected. Hippocampal formation atrophy was identified in three of the 15 patients. Neuroimaging-detected hippocampal formation atrophy has been shown to be a reliable marker of moderate to severe mesial temporal sclerosis. All patients with hippocampal formation atrophy had an unfavorable operative outcome. Pathological examination of the hippocampus in one patient with neuroimaging-identified hippocampal formation atrophy who subsequently received an anterior temporal lobectomy revealed mesial temporal sclerosis. Nine of the 12 patients without hippocampal formation atrophy experienced a significant reduction in seizure tendency after lesionectomy. The surgically excised hippocampus in one patient without hippocampal formation atrophy who later underwent a temporal lobectomy showed no significant neuronal loss. Results of this study have modified the surgical approach taken at this institution in patients with temporal lobe lesional epilepsy. Patients with magnetic resonance imaging-defined dual pathology now undergo a temporal lobectomy which includes resection of the hippocampus and the foreign-tissue lesion.