A 65-year old patient presented with newly diagnosed hypercalcemia and elevated parathyroid hormone levels. Exploration of parathyroid glands was performed under the diagnosis of primary hyperparathyroidism and was combined with thyreoidectomy due to concomittant multinodular goiter. However, no parathyroid adenoma was identified despite careful exploration. Therefore, the operation was terminated and an ectopic adenoma was suspected. A (99m)Tc-sestamibi scintigraphy with SPECT, neck ultrasound, neck CT scan with contrast media and selective venous sampling were performed for further localization of the adenoma. A single adenoma localized in the submandibular area of the left jaw was suspected and confirmed during surgical reexploration. Postoperatively, parathyroid hormone levels were below detection level and the patient required short-term calcium supplementation for symptomatic hypocalcemia.