It is controversial whether isolated lesions of mammillothalamic tract (MTT) produce significant amnesia. Since the MTT is small and adjacent to several important structures for memory, amnesia associated with isolated MTT infarction has been rarely reported. We report a patient who developed amnesia following an infarction of the left MTT that spared adjacent memory-related structures including the anterior thalamic nucleus. The patient s memory deficit was characterized by a severe anterograde encoding deficit and retrograde amnesia with a temporal gradient. In contrast, he did not show either frontal executive dysfunction or personality change that is frequently recognized in the anterior or medial thalamic lesion. We postulate that an amnesic syndrome can develop following discrete lesions of the MTT.