Background Patients with transfusion-dependent beta-thalassemia major (TM) are at risk for myocardial iron overload and cardiac complications. which was evaluated by a smoothness index of QTc (SI-QTc) a standard deviation of QTc (SD-QTc) and a QTc dispersion. Left ventricular function and myocardial T2* values were assessed by cardiac Klf2 magnetic resonance. Patients with TM experienced significantly greater SI-QTc SD-QTc and QTc dispersion compared to the control subjects (all p values<0.001). Spatial repolarization heterogeneity was even more pronounced in patients with significant iron overload (T2*<20 ms n?=?20) compared to those with normal T2* (all p values<0.001). Loge cardiac T2* correlated with SI-QTc (r?=??0.609 p<0.001) SD-QTc (r?=??0.572 p<0.001) and QTc dispersion (r?=??0.622 p<0.001) while all these indices had no relationship with measurements of the left ventricular geometry or function. At the time of study 10 patients experienced either heart failure or arrhythmia. All 3 indices of repolarization heterogeneity were related to the presence of adverse cardiac events with areas under the receiver operating characteristic curves (ranged between 0.79 and 0.86) similar to that of cardiac T2*. Conclusions Multichannel MCG exhibited that patients with TM experienced increased spatial repolarization heterogeneity which is related to myocardial iron weight and adverse cardiac events. Introduction Thalassemia major (TM) is a single gene disorder which results in severely impaired synthesis of the globin chain. Adoption of transfusion regimens and iron-chelating therapy has led to great improvement in life expectancy in the majority of affected individuals -. However cardiac iron overload resulting in heart failure and arrhythmia remains the major cause of death . Iron toxicity to cardiomyocytes not only results in impaired contractility   but also causes delayed electrical conduction and increased electrophysiological heterogeneities -. Numerous electrocardiographic parameters have been investigated in patients with TM including QTc interval QT dispersion and QT variability -. However the clinical implications of these parameters remain unclear. Magnetocardiography (MCG) is usually a medical technique that directly measures the poor magnetic field generated by electrical activity in the heart using a superconducting quantum interference device (SQUID). With the improvements in multichannel MCG systems this technique has been acknowledged for its outstanding ability to detect GSI-IX coronary artery disease and cardiac allograft vasculopathy after heart transplantation through the evaluation of various repolarization indices -. Compared to the 12-lead surface electrocardiogram (ECG) more registration sites in multichannel MCG allow not only a more sensitive GSI-IX calculation of the range of QT intervals but GSI-IX also a detailed examination of the spatial distribution of QT intervals over the heart. GSI-IX Therefore the purpose of this study was to investigate the applicability of MCG for detecting repolarization heterogeneity in patients with TM. Additionally we assessed the hypothesis that MCG-derived indices of spatial repolarization GSI-IX heterogeneity are related to cardiac iron weight and adverse cardiac events. Methods Ethics Statement The study protocol was approved by the institutional review table of the National Taiwan University Hospital and written informed consent from all participants was obtained. This study conformed to the principles of the Helsinki Declaration. Subjects From June 2008 to January 2010 patients with transfusion-dependent TM treated at the Pediatric Hematology Department of National Taiwan University Hospital served as the eligible population of this study. Patients were transfused every 2 to 4 weeks to keep hemoglobin levels above 10 g/dL. Among the 53 patients enrolled 3 were excluded from further analysis because of the presence of a conduction GSI-IX disturbance in 12-lead surfance ECG (QRS period >120 ms). Therefore a total of 50 patients with TM were analyzed. The control group for MCG consisted of 55 healthy subjects with comparable age and sex as the patient group. Control subjects were free from any cardiovascular disorder and experienced no conduction disturbance after evaluation by 12-lead surface ECG. Acquisition of SQUID MCG All participants were assessed by the 64-channel SQUID MCG system (developed by Korea Research Institute of Requirements and Science) to detect.