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Background: Whether Parkinson disease (PD) and dementia with Lewy bodies (DLB) represent 2 distinct nosologic entities or are diverse phenotypes of Lewy body disease is subject to debate., Objectives: To determine the accuracy of the diagnoses of Lewy body disease, PD, and DLB by validating the clinical diagnoses of 6 neurologists with the neuropathologic findings and to identify early predictors of the diagnoses., Methods: Six raters who were unaware of the neuropathologic diagnoses analyzed 105 clinical vignettes corresponding to 29 cases of Lewy body disease (post hoc analysis of 15 patients with PD and 14 with DLB) and 76 patients without PD or DLB whose cases were confirmed through autopsy findings., Main Outcome Measures: Sensitivity and positive predictive value (PPV) were chosen as validity measures and the K statistic as a reliability measure., Results: Interrater reliability for the diagnoses of Lewy body disease and PD was moderate for the first visit and substantial for the last, whereas agreement for diagnosis of DLB was fair for the first visit and slight for the last. Median sensitivity for diagnosis of Lewy body disease was 56.9% for the first visit and 67.2% for the last; median PPV was 60.0% and 77.4%, respectively. Median sensitivity for the diagnosis of PD was 73.3% for the first visit and 80.0% for the last; median PPV was 45.9% and 64.1%, respectively. Median sensitivity for the diagnosis of DLB was 17.8% for the first visit and 28.6% for the last; median PPV was 75.0% for the first visit and 55.8% for the last. The raters' results were similar to those of the primary neurologists. Several features differentiated PD from DLB, predicted each disorder, and could be used as clinical pointers., Conclusions: The low PPV with relatively high sensitivity for the diagnosis of PD suggests overdiagnosis. Conversely, the extremely low sensitivity for the diagnosis of DLB suggests underdiagnosis. Although the case mix included in the study may not reflect the frequency of these disorders in practice, limiting the clinical applicability of the validity measures, the raters' results were similar to those of the primary neurologists who were not exposed to such limitations. Overall, our study confirms features suggested to predict these disorders, except for the early presence of postural imbalance, which is not indicative of either disorder.