Introduction: The underlying pathophysiology of idiopathic REM sleep behavior disorder (iRBD) is not fully understood, although the condition is currently recognized as an early-stage alpha-synuclein disorder. We evaluated the morbidity, mortality, and rate of conversion to a neurodegenerative disorder in a national group of patients. Methods: All patients in Denmark with a diagnosis of RBD between 2006 and 2013 were identified from the Danish National Patient Registry (NPR) records. We excluded patients who had received a diagnosis of narcolepsy or any of the following neurodegenerative diseases before their diagnosis of RBD: Parkinson's disease, multiple system atrophy, progressive supranuclear paralysis, Alzheimer's, and Lewy body dementia. We used randomly chosen controls matched for age, gender, and municipality. Results: In total, 246 iRBD patients and 982 matched controls were analyzed. The mortality rate was the same in both groups. The morbidity rate was significantly higher in the years before and after an RBD diagnosis, due to a wide variety of disorders in the following major disease groups: mental/behavioral disorders; endocrine/metabolic diseases; diseases of the eye; diseases of the nervous, digestive, musculoskeletal, circulatory, and respiratory systems; abnormal findings not classified elsewhere; external causes; and factors influencing health status. The conversion rate from RBD to a neurodegenerative disease was 13% over the 8 years after a diagnosis of RBD. Conclusions: A diagnosis of RBD is associated with increased morbidity several years before and after a diagnosis is made. Patients have a higher risk of converting to a neurodegenerative disorder than matched controls. Mortality rates are unchanged.