Over a 25-year period, two carcinomas of the parathyroid were observed in 67 cases of primary hyperparathyroidism. The most important signs and symptoms were bone disease, palpable neck metastases, renal stones, and hypercalceamia with high blood levels of parathyroid hormone. Histology revealed that in principle parathyroid carcinoma can be distinguished from adenoma by a trabecular pattern and thick fibrous bands. The presence of cellular atypia and variation or mitotic figures (regressive polymorphia) was not a useful criteria for carcinoma. Local recurrence occurred in both cases.