Three patients with clinical suspicion of pheochromocytoma were studied with metaiodobenzylguanidine (MIBG) labeled with I-123 or I-131. In these three patients, marked uptake of the radiopharmaceutical was demonstrated in tissues that were subsequently proven not to be of neural crest origin. Furthermore, extensive biochemical evaluations and long-term followup definitively excluded pheochromocytoma in all three. Therefore, these three cases represent unexpected "false-positive" MIBG uptake.