It is not known whether lesions of the mammillary body per se can produce significant memory impairment in humans. The amnesia reported in patients with lesions in the mammillary bodies may have been produced by lesions coexisting in other memory-related areas, such as the medial thalamus, mammillothalamic tract, descending columns of the fornix, or amygdalofugal pathways. We describe a patient who developed anterograde amnesia following damage to the basal hypothalamus involving the mammillary bodies. The cause was a cystic craniopharyngioma. After surgical removal of the tumor, his memory function improved but remained considerably impaired, especially on delayed recall. Postsurgical MRI revealed small, atrophic mammillary bodies without residual tumor. There was no obvious evidence of damage to the hypothalamus. We therefore conclude that our patient's amnesia resulted from mammillary body lesions alone.