REM sleep behavior disorder (RBD) may occur acutely or subacutely, irrespective of age and sex. RBD or isolated REM sleep without atonia (RSWA) has been associated with various medications, or substance use/abuse, or abrupt withdrawal states. Less frequently, structural brain lesions (vascular, demyelinating disease, tumors), especially in the pontine region, may cause acute RBD. Also, RBD can appear acutely after a stressful life event and in post-traumatic stress disorder. RBD recognition in all these clinical conditions has a clinical value, but could have also a crucial importance in understanding the pathophysiology of RBD. More awareness of the existence of acute forms of RBD and greater familiarity with the various potential clinical pictures may help to avoid misdiagnoses and mistreatments. In addition, if prospective studies will confirm the existence of separate RBD patient subgroups (e.g., older men with neurodegenerative diseases, younger patients with narcolepsy, middle-aged women with depression taking antidepressants, and patients with autoimmune diseases), then prognostic differences and optimal treatments across RBD subgroups could emerge.