The radiopharmaceutical 123-I-mIBG has been approved for assessing sympathetic activity in neuroendocrine tumors and is currently undergoing investigation in the study of sympathetic innervations of the heart in heart failure and a variety of other electrophysiologic abnormalities of the heart, in which sympathetic innervations may be playing a role. The subject will be exposed to small doses of radiation during the course of the scan. Nuclear medicine has been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure. Allergic reactions to radiopharmaceuticals may occur but are extremely rare and are usually mild. This risk will be minimized by excluding individuals with previous allergic reactions to iodine. Injection of the radiotracer may cause slight pain and redness which should rapidly resolve. Some medications may have to be stopped a few hours before the 123-I-mIBG scan is performed.