Memory performance of patients with the clinical diagnosis of Alzheimer's disease was compared with performance of patients with alcoholic Korsakoff's syndrome and patients with Huntington's disease. Although all patient groups exhibited impairment on tests of verbal memory, only patients with Alzheimer's disease exhibited impaired priming. Priming is an unconscious expression of recently encountered material, and it is intact even in severely amnesic patients. Because mildly demented patients with Alzheimer's disease exhibited impaired priming, damage to brain structures in addition to those damaged in the amnesic syndrome must occur at a relatively early stage of the disease process.