Diabetes mellitus is a risk factor for cardiovascular morbidity and death. Cardiac autonomic diabetic neuropathy (CADN) is a common and serious complication of diabetes mellitus, which consists of damage to the autonomic nerve fibres that innervate the heart and blood vessels, resulting in alterations in heart rate control and vascular dynamics, associated with disabling clinical manifestations and increased mortality and incidence of both silent myocardial ischaemia and infarction. In vivo non-invasive radionuclide assessment of cardiac sympathetic innervation is possible with the use of radioabelled analogues of norepinephrine, which are actively taken up by the postganglionic presynaptic sympathetic nerve fibres of the heart. The catecholamine analogue 123I-metaiodobenzylguanidine (123I-MIBG) is the most commonly used tracer for imaging myocardial presynaptic sympathetic innervation on a broad clinical basis. 123I-MIBG imaging provides unique insights into the effects of diabetes on cardiac sympathetic integrity and the pathophysiological consequences of cardiac sympathetic dysinnervation. It also allows the assessment of both glycemic control on the progression of autonomic neuropathy and the effects of the autonomic derangement on myocardial blood flow regulation, function and electrophysiology, showing even potential for the prediction of adverse outcome.