Parathyromatosis is an uncommon cause of hypercalcemia. We describe a case of parathyromatosis and hypercalcemia in a 38-year-old man with a history of end-stage renal disease and subtotal parathyroidectomy in which a previous parathyroid operation was implicated. The results of fine-needle aspiration and histologic examination demonstrated a neck mass consistent with parathyromatosis. We discuss parathyromatosis as a rare cause of recurrent hypercalcemia in patients with end-stage renal disease and in those who have undergone previous parathyroid operations. We also characterize the iatrogenic and the embryologic pathogenetic factors involved.