We report a case of REM sleep behavior disorder (RBD) in a patient with spinocerebellar ataxia type 3 (SCA-3). In RBD, the muscle atonia normally prevailing during REM sleep is absent, and motor activity occurs within the context of vivid and violent dream mentation. The prevalence of RBD in the general population has been reported as 0.5%, 5 but is more commonly seen in multiple system atrophy, PD, dementia with Lewy bodies, collectively referred to as "synucleinopathies," as compared to other neurodegenerative disorders. 1 An increased prevalence of restless legs syndrome/periodic limb movements of sleep has also been reported in SCA-3. 2 To our knowledge, RBD has not been previously reported in SCA-3 patients.Case report. A 61-year-old man with a genetically proven diagnosis of SCA-3 (70 CAG repeats in the MDJ1 gene on chromosome 14q) presented with a 1-year history of at least five dream enactment episodes. He described having leapt from bed on two occasions while asleep without sustaining significant injuries. He reported violent dream mentation on awakening. These episodes were sporadic and without any set pattern during a particular part of the night. Dream enactment behaviors resolved with clonazepam 0.5 mg at bedtime.His gait instability started in his late twenties and he has been wheelchair bound for the past year and a half. He has a family history of this disease, with his mother, maternal aunt, and uncle being affected.His medical history included aortic aneurysm repair, mechanical heart valve, hypertension, glaucoma, and obstructive sleep apnea (OSA).Medications included carisoprodol, hydrocodone, enalapril, warfarin, sertraline, timolol gtt, and tolterodine.General physical examination was relevant only for mechanical auscultation of S2. Language, attention, and memory were normal. There was mild ataxic dysarthria. The fundi appeared normal. There were cataracts bilaterally. Extraocular muscle movements showed a bilateral abduction deficit and some skew in up and down gaze. Overshooting of saccades and restriction of upward gaze were noted. Muscle tone was normal. There was mild atrophy in his right-hand intrinsic muscles. He was strong in all four extremities except for minimal distal weakness in the lower extremities. Deep tendon reflexes were two and symmetrical, except for absent ankle reflexes and asymmetrical biceps, left greater than right. Plantar response was equivocal bilaterally. He had numbness in a stocking and glove distribution. Position and vibration sense were impaired in the lower extremities.Standard diagnostic video-polysomnography (PSG) failed to document parasomniac behavior diagnostic of RBD. A moderate degree of OSA with a respiratory disturbance index of 21.1/h and a periodic leg movement index of 5.4/h was also documented. Most remarkable was REM sleep-specific phasic-electromyographic (EMG) activity in the mentalis, left and right flexor carpi ulnaris, and tibialis anterior muscles. According to the International Classification of Sleep Disorders, minimal diagnostic crit...