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Background: Iodine-123 metaiodobenzylguanidine ((123)I-MIBG) lung uptake in the early phase has been proposed as a potential marker of endothelial function because MIBG behaves qualitatively similarly to norepinephrine in pulmonary circulation., Objectives: The purpose of the present study was to examine the lung uptake of (123)I-MIBG in patients diagnosed with myeloperoxidase antineutrophil cytoplasmic antibodies (MPO-ANCA)-associated vasculitis without clinical or radiological abnormalities of the thorax., Methods: Six patients with MPO-ANCA-associated vasculitis were enrolled. They had severe renal damage (mean creatinine: 5.1 mg/dl, mean blood urea nitrogen: 54.6 mg/dl), but no respiratory symptoms clinically or radiographic findings on chest computed tomography. The total lung to upper mediastinum ratio of (123)I-MIBG uptake (L/M) 15 min after the injection was measured. The result was compared with those for 6 patients with renal damage due to other diseases (mean creatinine: 6.2 mg/dl, blood urea nitrogen: 51.7 mg/dl) and for 8 healthy subjects., Results: The mean value of L/M in patients with MPO-ANCA-positive vasculitis was 1.21 +/- 0.04, which was significantly less than that of other groups (1.41 +/- 0.06 for patients with renal failure and 1.45 +/- 0.03 for normal volunteers). There were no significant differences in MIBG accumulation in the heart among the groups., Conclusions: The reduction in kinetic behavior of MIBG in the lung reflects the presence of pulmonary endothelial impairment in patients with MPO-ANCA-associated vasculitis, even though there are no clinical manifestations in the lungs.