To investigate calcitonin secretion in primary hyperparathyroidism, basal and stimulated (3 mg Ca<sup>++</sup>/kg body weight/10 min) immunoreactive calcitonin plasma levels were studied before parathyroidectomy. Plasma calcitonin levels were raised in 50% of patients regardless of sex, but a significant correlation between basal plasma calcium and calcitonin was found only in males. A reduced calcitonin response to calcium infusion was observed in all patients. Parathyroidectomy invariably induced a normalization of calcitonin basal levels. Our findings confirm the existence of a decreased parafollicular cell reserve probably as a consequence of the persistent hypercalcemic state in hyperparathyroid patients and suggest that it is more frequent in females.