Between 1960 and April, 1980, 302 patients were explored for primary hyperparathyroidism at Emory University Hospital. Seventeen of these 302 patients had undergone initial surgical exploration elsewhere, and were referred for persistent hypercalcemia. Of the 285 patients who were operated on at our institution, 14 subsequently had persistent hypercalcemia, and two had recurrent hypercalcemia. Twenty-eight of these 33 patients had had re-exploration, and 23 (82%) are now normocalcemic. Twenty-eight abnormal glands were found; 22 (79%) were retrievable via the neck and six (21%) required sternotomy. Of those glands removed via the neck, nine were in a near normal location and 13 in a subnormal or abnormal location. The causes of initial surgical failures were abnormally located glands, in ten patients, insufficient explorations of the neck in eight patients, hyperfunctioning parathyroid remnants in three patients, inadequate plans for hyperplasia in two patients, and carcinoma in one patient. In 57% of our patients who underwent successful re-exploration, the glands were correctly localized before operation by angiographic examination or selective venous sampling for parathormone. Thorough exploration and obtainment of biopsy specimens of all parathyroid glands are recommended in order to keep initial failures to a minimum. If reoperation is required, localization using CT scan, angiography and selective venous sampling are recommended.