In most cases, abnormal cardiac 123I-meta-iodobenzylguanidine (MIBG) scintigraphy increases the probability of a diagnosis of Parkinson's disease (PD) in patients with parkinsonian features. In our study, we validated the additional value of 123I-MIBG scintigraphy beyond providing information on neurological findings and response to dopaminergic therapy for the diagnosis of PDin the early phase. We investigated 77 cases of PD (Hoehn and Yahr Stages I-III) and 73 cases of atypical parkinsonian disorder (APD), including 35 patients with multiple system atrophy, 19 with corticobasal syndrome, and 19 with progressive supranuclear palsy. Two multiple logistic regression models were developed to predict the probability of PD based on APD. Common covariates were resting tremor, vertical supranuclear palsy, apraxia, cerebellar symptoms, and response to dopaminergic therapy with MIBG scintigraphy (reference model) or without it (MIBG-added model). The net reclassification index (NRI) was examined and net benefit using decision curve analysis was performed to examine the additional clinical value of MIBG scintigraphy. Finally, we estimated the cost-effectiveness of MIBG scintigraphy. The MIBG-added model significantly improved the ability to classify PD or APD compared with the reference model (NRI index 1.390, p < 0.001). However, the decision curve of the reference model ranked equally with the MIBG-added model up to a risk threshold of 0.8. In addition, MIBG scintigraphy was not cost-effective. Although MIBG scintigraphy has statistical usefulness for PD diagnosis, there may be little additional benefit in the early phase of PD beyond the neurological findings and response to dopaminergic therapy regarding clinical effectiveness and cost-effectiveness. It may be of greatest value when neurological findings that do not match PD are observed during the clinical course.