Serious arrhythmias, sometimes related to the injection of iodinated contrast media, are known complications of cardiac angiography. A new, iodine-based, non-ionic, iso-osmolar x-ray contrast media is in development for use in these procedures. This contrast medium, iosimenol, has a lower viscosity, higher electrolyte content, and higher iodine concentration than other available iso-osmolar contrast media. The present study is a retrospective re-read and centralized analysis of the electrocardiographic response to intravenous and non-cardiac intraarterial injections of iosimenol, placebo, or iodixanol (Visipaque; GE Healthcare, Inc) in a total of 167 healthy subjects and patients enrolled in early clinical trials. No clinically relevant changes in heart rate and rhythm, morphology, atrioventricular conduction, or ventricular repolarization were noted after injection of any of the test articles in these studies. These results, despite the limited number of patients in these trials, suggest that iosimenol can be used safely in larger populations.