Aim Single photon emission computed tomography (SPECT) studies showed that regional cerebral blood flow (rCBF) abnormalities in idiopathic rapid eye movement (REM) sleep behavior disorder (iRBD) are similar to those seen in Parkinson's disease (PD) and dementia with Lewy bodies (DLB). The aim of the present study was to assess the longitudinal rCBF changes in patients with iRBD using repeated SPECT. Methods Nine patients with iRBD (7 men and 2 women; mean age 71.1 ± 3.2 years) underwent baseline and follow‐up SPECT studies (a mean interval of 22.8 ± 9.2 months). Results A decrease in rCBF was found in bilateral parietotemporal and occipital areas at the first and second SPECT. Compared with the first SPECT, the second SPECT showed a decreased rCBF in the medial portions of the parietooccipital lobe with a significant decrease in rCBF of the right posterior cingulate. None of the patients showed any neurological deficits, including extrapyramidal and cerebellar signs, visual hallucinations, and neuropsychological impairments during the study. Conclusion These findings suggest that longitudinal measurements of rCBF can show the presence of progressing neurodegenerative process in iRBD. Longitudinal SPECT study can be used to monitor the progression of degenerative process in patients with iRBD, even though there were no evolving neurological and neuropsychiatric impairments. Geriatr Gerontol Int 2013; ●●: ●●–●●.