Bilateral medial temporal lobe resection in man results in a persistent impairment of recent memory whenever the removal is carried far enough posteriorly to damage portions of the anterior hippocampus and hippocampal gyrus. This conclusion is based on formal psychological testing of nine cases (eight psychotic and one epileptic) carried out from one and one-half to four years after operation. The degree of memory loss appears to depend on the extent of hippocampal removal. In two cases in which bilateral resection was carried to a distance of 8 cm posterior to the temporal tips the loss was particularly severe. Removal of only the uncus and amygdala bilaterally does not appear to cause memory impairment. A case of unilateral inferior temporal lobectomy with radical posterior extension to include the major portion of the hippocampus and hippocampal gyrus showed no lasting memory loss. This is consistent with Milner and Penfield's negative findings in a long series of unilateral removals for temporal lobe epilepsy. The memory loss in these cases of medial temporal lobe excision involved both anterograde and some retrograde amnesia, but left early memories and technical skills intact. There was no deterioration in personality or general intelligence, and no complex perceptual disturbance such as is seen after a more complete bilateral temporal lobectomy. It is concluded that the anterior hippocampus and hippocampal gyrus, either separately or together, are critically concerned in the retention of current experience. It is not known whether the amygdala plays any part in this mechanismi, since the hippocampal complex has not been removed alone, but always together with uncus and amygdala.