Borrelia spirochetes were directly visualized in autopsy brain tissue from a patient with Alzheimer's disease and were cultured from cerebral cortex in artificial media. The authors propose that, as occurs in tertiary neurosyphilis and general paresis of the insane, Borrelia species may invade the brain, remain in a latent state for many years, and cause dementia in the absence of other focal neurologic deficits. An undetermined fraction of patients with Alzheimer's disease may be shown to have late tertiary neuroborreliosis.