A patient with an asymptomatic nonsecreting left adrenal pheochromocytoma concentrated I-123 MIBG. Iodine-123 MIBG scintigraphy showed abnormal uptake in the left adrenal bed. The authors conclude that I-123 MIBG imaging may allow the in vivo diagnosis of non-functioning pheochromocytomas when there is no clinical evidence of disease and other laboratory measurements are not helpful for tumor diagnosis.