SummaryAim: Cardiac 123I metaiodobenzylguanidine (MIBG) imaging can be influenced by several factors. We evaluated the relationship between catecholamine measurements and cardiac 123I MIBG uptake in neuroblastoma patients. Patients, methods: 30 neuroblastoma patients were retrospectively assessed on cardiac 123I MIBG uptake and urinary catecholamine dopamine and metabolites, homovanillic acid (HVA) and vanillylmandelic acid (VMA). Cardiac 123I MIBG uptake was quantified by heart-to-mediastinum (H/M) ratios, which were calculated into standard deviation scores (SDS) using age-specific reference values. Results: In 17 (57%) and 12 patients (40%) H/M ratio measurements were below –1.0 and –2.0 SDS at diagnosis. A significant inverse correlation between the average of urine metabolites HVA and VMA, and H/M ratio SDS was observed (r -.39, p = 0.04). Furthermore, there was a significant correlation between the urinary catecholamine metabolite HVA and H/M ratio SDS (r -.40, p = 0.04). Conclusion: Routine calculation of H/M ratios in 123I MIBG scintigrams of neuroblastoma patients is not helpful because it will not identify cardiac ventricular dysfunction in this patient category. A low H/M ratio on 123I MIBG scintigraphy is explained by increased cathecholamine levels secreted by neuroblastoma tumours.