Of 13 patients treated surgically for familial medullary thyroid carcinoma in whom parathyroid tissue was available, the majority showed parathyroid abnormalities (hyperplasia in six, tumors in five). Two patients had had renal calculi. No correlation was evident between the presence of the parathyroid tumors and peripheral blood levels of parathyroid hormone. Hyperparathyroidism is usually mild, but occasionally it results in complications of hypercalcemia. Hyperparathyroidism has not appeared to date following removal of medullary thyroid carcinoma associated with normal-sized but microscopically hyperplastic parathyroids. Evidence of parathyroid abnormalities has not been recognized in eight patients with sporadic medullary carcinoma, making genetic factors dominant in explaining the association of parathyroid hyperplasia and this carcinoma. At operation, parathyroid glands should be evaluated and those that are grossly enlarged removed while preserving parathyroid function.