Objective: To determine whether the Parkinson disease-related covariance pattern (PDRP) expression is abnormally increased in idiopathic REM sleep behavior disorder (RBD) and whether increased baseline activity is associated with greater individual risk of subsequent phenoconversion.Methods: For this cohort study, we recruited 2 groups of RBD and control subjects. Cohort 1 comprised 10 subjects with RBD (63.5 6 9.4 years old) and 10 healthy volunteers (62.7 6 8.6 years old) who underwent resting-state metabolic brain imaging with 18 F-fluorodeoxyglucose PET. Cohort 2 comprised 17 subjects with RBD (68.9 6 4.8 years old) and 17 healthy volunteers (66.6 6 6.0 years old) who underwent resting brain perfusion imaging with ethylcysteinate dimer SPECT. The latter group was followed clinically for 4.6 6 2.5 years by investigators blinded to the imaging results. PDRP expression was measured in both RBD groups and compared with corresponding control values.Results: PDRP expression was elevated in both groups of subjects with RBD (cohort 1: p , 0.04; cohort 2: p , 0.005). Of the 17 subjects with long-term follow-up, 8 were diagnosed with Parkinson disease or dementia with Lewy bodies; the others did not phenoconvert. For individual subjects with RBD, final phenoconversion status was predicted using a logistical regression model based on PDRP expression and subject age at the time of imaging (r 2 5 0.64, p , 0.0001).Conclusions: Latent network abnormalities in subjects with idiopathic RBD are associated with a greater likelihood of subsequent phenoconversion to a progressive neurodegenerative syndrome.