Tertiary hyperparathyroidism is considered as an autonomous proliferation state of the parathyroid glands with biological hyperfunction resistant to calcium/vitamin suppressor therapy. This phenomenon is thought to be secondary to monoclonal inactivation of tumoral growth suppression factor located on chromosome 11. Three patients, 13, 15, and 22 years of age, with chronic renal insufficiency of long evolution who presented with tertiary hyperparathyroidism following renal transplantation are described. The three patients underwent subtotal parathyroidectomy with consequent normalization of biochemical parameters of phospho-calcium metabolism in the first few weeks post surgery. Pathologic study showed adenoma in the affected glands with hyperplasia of the rest. We believe that in patients with long-term renal insufficiency an aggressive treatment, either medical or surgical, of secondary hyperparathyroidism which is continued after renal transplantation may be useful in preventing the development of tertiary hyperparathyroidism.