The Letter to Editor is a response to the recent paper by Li et al. (2018) on the prospective memory (PM) impairment in idiopathic REM sleep behavior disorder (iRBD). It shows conclusively the important pieces of information that were not cited by Li et al. (2018) and presented in our study (Bezdicek et al., 2018) that chronologically preceded the findings by Li et al. (2018).