A case of primary hyperparathyroidism due to parathyroid adenoma is presented. A 14-y-o male came in with voiding difficulty. The intravenous pyelogram (IVP) demonstrated a stone at the proximal one third of the left ureter and marked hydronephrosis of the left kidney, and the Tc-99m sestamibi nuclear scan demonstrated an hot spot below the lower pole of the left lobe of the thyroid. Laboratory study demonstrated hypercalcemia (12.4mg/dL) and elevated parathyroid hormone (143.67pg/mL). The excised parathyroid gland which was located below the lower pole of the left lobe of the thyroid, turned out to be parathyroid adenoma, consisting of mainly chief cells. Postoperatively he complained of transient hypocalcemic symptoms, which resolved by administration of calcium preparation and vitamin D.