In patients with Alzheimer's disease (AD), greatly diminished REM sleep might be expected because of the cholinergic deficit in this disease and because cholinergic agonists stimulate REM sleep in humans and animals. We present here an unusual case of neuropathologically verified AD with abundant REM sleep. We suggest 4 possible explanations for this phenomenon: (1) selective cell loss in caudal midbrain/rostral pontine structures known to control sleep; (2) development of narcolepsy; (3) unrecognized affective disorder; (4) disruption of circadian timekeeping system.