In order to investigate whether REM sleep without atonia (RWA) is detected even at early stage of sporadic olivopontocerebellar atrophy (OPCA), all-night polysomnography with video monitoring was performed on 5 patients presenting with no nocturnal behavioral complaints. RWA associated with sleep talk and/or minor movements in the head, neck, face and extremities was documented in all cases. Preserved proper NREM/REM cycle and relatively normal sleep architecture suggest the abnormality is confined to REM sleep. Dysfunction of the mechanisms underlying muscle atonia in REM sleep, especially functional or anatomical interruption of ponto-medullary pathways mediating REM sleep atonia, seems to cause RWA in these patients. RWA may be a clinically important finding as a sign of brainstem dysfunction at early stage of neurodegenerative diseases.