Background: Rapid eye movement (REM) sleep behavior disorder (RBD) has been described predominantly in elderly men and in association with neurodegenerative disease. But an increasing proportion of cases in recent reports and in clinical practice do not fit this description; thus we sought to describe a current RBD population and possibly identify new subgroups with RBD. Methods: Records of 115 consecutive patients with polysomnogram-confirmed RBD at an academic sleep center were retrospectively reviewed. Results: Male to female ratio was 2:1, and 1.25:1 for early-onset (age <50) cases. Mean age at diagnosis was 53.7 ± 16.4. years. Most (60%) cases were idiopathic, and neurodegenerative disease was coincident primarily in older men. Autoimmune disease was unexpectedly common in women (20%) particularly in the 30-49 age groups (40%). Antidepressant use was frequent (46.1%), especially in early-onset cases (57.8%). Conclusions: RBD is diagnosed more equally between men and women and in younger individuals than previously reported. While neurodegenerative disease is frequently co-incident with RBD in older men, most women and early-onset cases have " idiopathic" RBD. High prevalence of autoimmune disease among women with RBD suggests an intriguing link between immune dysfunction and RBD. A high rate of antidepressant use provides support for a potentially causal role for antidepressants in RBD.