The accurate diagnostic localization of extra-adrenal pheochromocytoma is important in the treatment of primary and metastatic tumors. In this study, we report on a case of extra-adrenal pheochromocytoma in which the tumor could not be diagnosed by 131I-metaiodobenzylguanidine (MIBG) scintigraphy but only by 123I-MIBG scintigraphy. Our patient was a 16-year old girl whose chief complaint was headaches. High levels of catecholamines were detected in the blood and urine samples. 131I-MIBG scintigraphy could not detect any abnormal accumulation, but an accumulation of 123I-MIBG was found in the pelvis. Magnetic resonance imaging (MRI) revealed a broad-based bladder tumor at the site of the accumulation. Partial cystectomy was performed, and immunohistochemical staining examination confirmed the diagnosis of extra-adrenal pheochromocytoma in the urinary bladder. In this report, we compare 131I-MIBG scintigraphy with 123I-MIBG scintigraphy and discuss their characteristics.