To analyse the presence and extent of global and regional distributions of cardiac sympathetic dysinnervation in long-term insulin-dependent diabetes mellitus (IDDM) without myocardial perfusion abnormalities (99mTc-methoxy isobutyl isonitrile study), 123I-metaiodobenzylguanidine (123I-MIBG) scintigraphy was performed in two clinically-comparable groups (20 diabetic patients with and 22 diabetic patients without ECG-based cardiac autonomic neuropathy). For comparison nine control subjects without heart disease were investigated. Only six diabetic patients (27%) without and one diabetic patient (5%) with ECG-based autonomic neuropathy were found to have a uniform homogeneous uptake of 123I-MIBG, in contrast to a uniform homogeneous uptake in all control subjects. The uptake of 123I-MIBG in the posterior myocardium of diabetic patients was smaller than in the anterior, lateral and septal myocardium (p < 0.001, p < 0.001, p = 0.001). In addition, diabetic patients with cardiac autonomic neuropathy (> or = two of five age-related cardiac reflex tests abnormal) demonstrated a more reduced uptake in the global, lateral and posterior myocardium than diabetic patients without (p < 0.01, p < 0.01, p < 0.001). A correlation between global or regional myocardial 123I-MIBG uptake, however, and duration of diabetes, HbA1c, body mass index or QT interval length was not observed. Our study demonstrates that cardiac sympathetic dysinnervation is common in long-term IDDM even in patients without ECG-based cardiac autonomic neuropathy and that the posterior myocardium is predominantly affected. We conclude that 123I-MIBG scintigraphy is a promising approach to further elucidate the pattern and natural history of myocardial dysinnervation in IDDM.