Three aged patients (age 63-78 years) had 3-15-year histories of abnormal behaviors during nocturnal sleep. These three patients presented no other psychiatric problems. Polysomnographic recordings were carried out from 1-3 nights after adaptation. The patients were simultaneously monitored via a video system during the recording period. Various nocturnal behaviors, including laughing, weeping, the shaking of arms and getting up were detected during these recordings. These behaviors appeared exclusively during REM periods which were associated with a large amount of abnormal REM sleep lacking muscle atonia (d-STREM with EMG). NREM sleep architecture was intact and %SWS was within the normal limits per age for all the patients. The patients, manifesting such abnormal behaviors during sleep, could be aroused completely with awakening stimuli, and could recall in detail their unpleasant dreams. Two of the three were treated with clonazepam (0.5-1 mg/day) which immediately decreased the abnormal behaviors during sleep. Polysomnographic studies after the treatment showed a marked decrease in the d-STREM with EMG. The above results indicate that these abnormal behavior during nocturnal sleep were closely linked to d-STREM with EMG. And the administration of clonazepam not only led to an amelioration in the abnormal behaviors but also in the polysomnographic pathology. It is concluded that parasomnia in the aged is characterized by behavioral manifestations of the dreaming due to dysfunctions of the muscle atonia system.