From January 1993 to January 1994, scintigraphy with 123I-MIBG and/or 131I-MIBG were performed in 22 patients and their scintigraphic usefulness was evaluated. Iodine-123 MIBG and 131I-MIBG scintigrams were obtained 24 hours after injection of 222 MBq of 123I-MIBG and 48 hours after injection of 20 MBq of 131I-MIBG, respectively. In two patients with pheochromocytoma, the 123I-MIBG and 131I-MIBG scans were performed and both images were compared. In a patient with single intraadrenal pheochromocytoma, the lesion not detected with 131I-MIBG was clearly visualized with 123I-MIBG. In the other patient with multiple metastatic pheochromocytoma, much more lesions were distinctly demonstrated on the 123I-MIBG images than on the 131I-MIBG images. All of the lesions were detected with 123I-MIBG in a patient with pheochromocytoma, 3 patients with neuroblastoma and a patient with medullary thyroid cancer. Most of the normal adrenal glands (86%) were visualized on the 123I-MIBG scintigrams, in 7 patients without neural crest tumor and adrenal diseases, while 131I-MIBG scintigraphy failed to visualize normal adrenal glands in 2 hypertensive patients. The main reason for the superiority of 123I-MIBG to 131I-MIBG is considered to be as follows: 1) higher specific activity of 123I-MIBG. 2) the larger amount of 123I-MIBG used. 3) gamma ray energy of 123I is ideal for gamma camera. In conclusion, 123I-MIBG appears to be a more suitable imaging agent than 131I-MIBG in depicting neural crest tumors.