The identification of brain structures and connections involved in memory functions has depended largely on clinico-pathological studies of memory-impaired patients, and more recently on studies of a primate model of human amnesia. But quantitative neurobehavioural data and detailed neuropathological information are rarely available for the same patients. One case has demonstrated that selective bilateral damage to the hippocampus causes a circumscribed memory impairment in the absence of other intellectual deficits. This finding, in conjunction with evidence from humans and monkeys, indicates that the hippocampus together with adjacent and anatomically related structures is essential for the formation of long-term memory, perhaps by virtue of the extensive reciprocal connections between the hippocampal formation and putative memory storage sites in the neocortex. Although cognitive studies of amnesia provide useful information about the functional organization of normal memory, it has not usually been possible to relate memory impairment to anatomy in living patients. We have developed a high-resolution protocol for imaging the human hippocampus with magnetic resonance that permits visualization of the hippocampal formation in substantial cytoarchitectonic detail, revealing abnormalities in patients with severe and selective memory impairment.