The history, physical and radiologic findings, treatment and pathology in five unusual cases of hyperparathyroidism is presented. The hyperparathyroidism was caused by a large (113 grams) mediastinal adenoma in the first patient, who is alive 25 years after surgery. A parathyroid carcinoma with compression of the esophagus was documented in the second patient. This patient is alive and normocalcemic 23 years after surgical treatment. A third patient with hyperplasia returned with hypercalcemia 20 years postsurgery requiring reoperation. A fourth patient with advanced bone findings was found to have a parathyroid adenoma. The fifth case is a patient with tertiary hyperparathyroidism secondary to hypophosphatemic rickets.