Primary hyperparathyroidism is the most common cause of hypercalcemia in nonhospitalized patients. Recently, technetium-99m sestamibi (methoxyisobutylisonitrile, Tc-99m MIBI) imaging has been established as the best method for detecting a parathyroid adenoma. However, sestamibi accumulates in a number of nonparathyroid tissues. We report a patient with suspected primary hyperparathyroidism, who underwent sestamibi parathyroid imaging. In addition to sestamibi retention in a parathyroid adenoma, retention was observed in a breast fibroadenoma, which mimicked an ectopic parathyroid adenoma.