Although the guidelines of the Japanese Circulation Society recommend Iodine-123 meta-iodobenzylguanidine (123I-MIBG) myocardial scintigraphy in patients with heart failure, this scintigraphy can be applied to evaluate cardiac sympathetic activity in hypertrophic cardiomyopathy (HCM) and takotsubo syndrome (TTS). Increased cardiac sympathetic activity in HCM is associated with the various clinical features, the advancement of disease stage, and the prognosis. However, it has not been fully studied whether this neurotransmitter imaging function as a monitor of the therapeutic effect in HCM. 123I-MIBG myocardial scintigraphy has been taken an important role to explore the underlying pathophysiological mechanism in TTS. TTS is a novel cardiac syndrome involving acute onset of ST-segment elevation on electrocardiography, mimicking acute myocardial infarction without coronary artery flow limitation. Although over 25- years have passed since the first description, the details of the underlying pathophysiology are still unclear. Of great interest now is whether it could be utilized to stratify the cardiac risk in patients with TTS using 23I-MIBG myocardial scintigraphy. In this chapter, we focus on the clinical value of 123I-MIBG myocardial scintigraphy in these patients.