In 123I-metaiodobenzylguanidine (123I-MIBG) myocardial scintigraphy, the early heart-to-mediastinum (H/M) ratio is considered to reflect the density of the cardiac sympathetic nerve endings, washout rate (WR) is an indicator of the cardiac sympathetic tone, and the delayed H/M ratio reflects both. The Delayed H/M ratio is usually used to support the diagnosis of Lewy body diseases (LBDs) and idiopathic REM sleep behavior disorder (iRBD); however, which values should be used have not been specified. Here, we hypothesized that the combination of these values is appropriate for the diagnostic purpose.In this single-center retrospective cohort study, we recruited 106 patients with LBDs or iRBD and 33 patients without those diseases, of whom we reviewed the 123I-MIBG myocardial scintigraphy results.Sensitivity/specificity to diagnose LBDs and iRBD were 0.77/0.94 for the early H/M ratio (≤ 2.0), 0.82/0.94 for the delayed H/M ratio (≤ 2.0), and 0.89/0.91 for WR (≥ 23.0). When patients were considered positive if at least either the early H/M ratio or WR was abnormal, the sensitivity significantly increased to 0.97, whereas the specificity remained similar at 0.91. Furthermore, our subgroup analyses revealed that WR enhancement preceded H/M ratio reduction, but, in patients with a severely reduced early H/M ratio, paradoxically normal WR could be observed.We propose the highly sensitive, combined early H/M ratio and WR assessments for 123I-MIBG myocardial scintigraphy. The temporal precedence of cardiac sympathetic dysfunction over denervation and the floor effect in 123I-MIBG uptake may underlie the sensitivity improvement.