A 60-year-old patient with multiple system atrophy (MSA) who presented with rapid eye movement (REM) sleep behavior disorder was investigated longitudinally by all-night polysomnography. REM sleep components, i.e. rapid eye movements and chin muscle activity, were analyzed together with the frequency of behaviors/movements on the videorecording. Decreased frequency of elaborate motor activity during REM sleep with time in this patient was compatible with the observation by his wife, and this change seemed to correlate with predominant tonic chin electromyogram with relatively suppressed phasic chin muscle activity, but the reduction of the REM sleep behavior disorder (RBD) episodes could be interpreted as being due to the increased rigidity along with MSA progression. The chronological change in REM sleep components in RBD with neurological disorders is worth studying in large follow-up series to enlarge our knowledge about the mechanism of behavioral manifestation of RBD in humans.