Diabetic neuropathy is thought to comprise a reversible metabolic and an irreversible structural component of neuronal abnormality. To investigate whether the cardiac sympathetic denervation recently described in newly diagnosed, but metabolically stabilized, diabetic patients without myocardial perfusion abnormalities reflects transient or permanent sympathetic abnormalities, 123-I-metaiodobenzylguanidine (123-I-MIBG) scintigraphy was performed in 16 patients with insulin-dependent (Type 1) diabetes mellitus (IDDM) 1 year after initial assessment and diagnosis. All patients had been treated with an intensified insulin therapy for 1 year. HbA1c had fallen from 11.5 +/- 2.0% to 6.3 +/- 0.9% (p < 0.001). The global myocardial 123-I-MIBG uptake (score 1-6) had improved in 7 patients at 1 year, remained unchanged in 7, and deteriorated in 2 patients. Regionally, the myocardial uptake score of the posterior and septal regions had improved significantly (p < 0.01, p = 0.02) with a mean uptake score in the groups of 3.8 +/- 1.1 and 3.4 +/- 1.2 at diagnosis versus 2.6 +/- 0.5 and 2.5 +/- 0.9 at 1 year. Myocardial uptake scores of the anterior, lateral, and apical regions had also improved in 7, 6, and 9 patients, but the mean changes of these scores did not reach significance. The study demonstrates that scintigraphically assessed cardiac sympathetic denervation in newly diagnosed, but metabolically stabilized, IDDM patients is partially reversed with improved metabolic control after 1 year of intensified insulin therapy. We suggest that even in the early stage of IDDM, cardiac sympathetic dysfunction is composed of reversible and irreversible neuronal abnormalities.