Persisting hypercalcemia after surgery for primary hyperparathyroidism is a challenge for the surgeon: once non-parathyroid causes for the hypercalcemia have been excluded the only remaining possible causes are ineffective surgery and a failed surgery. Between 1986 and 1994, 414 patients with primary hyperparathyroidism were operated upon, 32 of whom presented with persisting hypercalcemia; 24 of these patients had their first operation in another hospital. The cause for persisting hypercalcemia was a single adenoma in 27 patients (84%), double adenoma in 2 patients and primary hyperplasia in 2 patients. In 1 patient an unsuccessful revision operation was performed. In 17 patients the parathyroid glands had an atypical position. There were 11 patients who presented with recurrent hyperplasia. The reasons for recurrence were double adenoma in 2 patients and hyperplasia in 8 cases (73%). In 1 patient no pathologic gland was found. Although preoperative localization studies were positive in only 32%, the overall surgical success rate fortunately amounted to a satisfactory 95%.