A 67-year-old woman who presented with hypocalcemia compatible with idiopathic hypoparathyroidism gradually changed into a state of primary hyperparathyroidism. The left upper parathyroid gland, which was larger and harder than other glands, was resected. Despite the operation, hypercalcemia and high levels of intact PTH persisted. Six weeks later total parathyroidectomy was done to induce remission. The resected gland in the first operation had clusters of lymphoid follicles with germinal centers indicating a chronic autoimmune inflammation. This case suggests a transition from hypoparathyroidim to hyperparathyroidism associated with chronic parathyroiditis, possibly by a mechanism analogous to that observed in chronic thyroiditis.