Rapid eye movement (REM) sleep behavior disorder (RBD) is a parasomnia defined by intermittent loss of electromyographic atonia during REM sleep with emergence of complex and vigorous behaviors. Punching, kicking, and leaping from bed during attempted dream enactment caused repeated injury in nine of our first ten adult patients. Mean age at onset was 62 years; nine of the patients were male. All patients underwent standard polysomnographic studies with videotaping of behaviors and extensive neurologic and psychiatric evaluations. The RBD was unrelated to psychopathologic conditions but in five cases was closely linked with major neuropathologic disorders: dementia (two), olivopontocerebellar degeneration, subarachnoid hemorrhage, and the Guillain-Barré syndrome. Other common polysomnographic abnormalities were high REM density, increased stage 3/4 (slow-wave) sleep, and both periodic and aperiodic limb twitching in non-REM sleep. Eight patients had dream changes involving motor overactivity and violent confrontations of dream characters. Clonazepam induced rapid and sustained improvement of dream and sleep behavior problems in seven patients, as did desipramine hydrochloride in one patient.