The efficacy of the newly developed pheochromocytoma-seeking radiopharmaceutical, [131I]MIBG, was examined in the first 400 patients (441 studies) investigated for suspected pheochromocytoma at our institution. The results of [131I]MIBG scintigraphy were classified as true positive, false positive, true negative, and false negative. Using this classification the sensitivity was found to be 78.4% in primary, sporadic pheochromocytoma, 92.4% in malignant pheochromocytoma, and 94.3% in familial pheochromocytoma giving an overall sensitivity of 87.4%. The specificity was 98.9% in primary, sporadic pheochromocytoma, 100% in malignant pheochromocytoma, and 100% in familial pheochromocytoma. The overall specificity was 98.9%. Iodine-131 MIBG scintigraphy was thus found to be a safe, noninvasive, and efficacious technique for the location of pheochromocytomas, especially for those arising from nonadrenal sites, recurring postoperatively, and exhibiting malignant metastatic disease. We find that, where available, [131I]MIBG scintigraphy is the study of choice to initiate the location of suspected pheochromocytoma.