Parathyroid carcinoma is a rare endocrine malignancy characterized by the exaggerated metabolic effects of the parathyroid glands. The preoperative differential diagnosis between parathyroid carcinoma and primary hyperparathyroidism is often difficult because many of the signs and symptoms are very similar. Intraoperative differentiation is obscured by the strict anatomic and histologic criteria required for diagnosis of parathyroid carcinoma. We have encountered three patients with parathyroid carcinoma during the last 10 years and managed them successfully. Two of them presented with recurrence of hypercalcemia, one 11 years after and the other 3 years after the primary operation for hyperparathyroidism; both patients were eventually diagnosed with parathyroid carcinoma. The third case was suspected as primary hyperparathyroidism preoperatively but confirmed as carcinoma subsequent to histologic examination.