BACKGROUND: Different studies have found diminished cardiac metaiodobenzylguanidine (MIBG) uptake in Lewy body (LB) related conditions (Parkinson's disease (PD) and Lewy body dementia (LBD)). However, delayed heart/mediastinum (d-H/M) ratio diagnostic cutoff points are debated in parkinsonian syndromes. METHODS: We performed a monocentric retrospective analysis on 62 consecutive parkinsonian patients who underwent an (123)I-MIBG scintigraphy, brain imaging and dopaminergic imaging using (123)I-Ioflupane single photon emission computed tomography (SPECT) from 2009 to 2013. The optimal d-H/M ratio was determined from a Receiver Operating Characteristic (ROC) curve and the sensitivity (Se), specificity (Sp) and likelihood ratios (LR) were calculated. 42 patients were diagnosed with LB diseases (20 PD, 22 LBD) and 20 patients with other diseases. RESULTS: (123)I-MIBG scintigraphy helped to distinguish PD (p < 0.001) and LBD (p = 0.03) from other diseases. The optimal d-H/M ratio was 1.48 (0.85 area under the ROC curve). Se and Sp were 83.3 %, and 85 % respectively with positive and negative LR of 5.5 and 0.2 respectively. Patients with LBD had a lower d-H/M ratio than patients with PD (result not statistically significant) and a cutoff point at 1.2 could help to differentiate the two diseases. We did not find any correlation between the d-H/M ratio and clinical or (123)I-Ioflupane SPECT data. CONCLUSION: According to our population, the d-H/M ratio at 1.48 led to the best performance diagnosis with good Se, Sp and accuracy. In addition, a d-H/M ratio cutoff at 1.2 could help to differentiate PD from LBD.